diff --git a/src/browserContext.ts b/src/browserContext.ts
index a1a66dd0bd..7d4944f1e3 100644
--- a/src/browserContext.ts
+++ b/src/browserContext.ts
@@ -20,7 +20,7 @@ import { helper } from './helper';
 import * as network from './network';
 import * as path from 'path';
 import { Page, PageBinding } from './page';
-import { TimeoutSettings } from './timeoutSettings';
+import { TimeoutSettings } from './utils/timeoutSettings';
 import * as frames from './frames';
 import * as types from './types';
 import { Download } from './download';
@@ -28,6 +28,7 @@ import { Browser } from './browser';
 import { EventEmitter } from 'events';
 import { Progress } from './progress';
 import { DebugController } from './debug/debugController';
+import { isDebugMode } from './utils/utils';
 
 export class Screencast {
   readonly path: string;
@@ -68,7 +69,7 @@ export abstract class BrowserContext extends EventEmitter {
   }
 
   async _initialize() {
-    if (helper.isDebugMode())
+    if (isDebugMode())
       new DebugController(this);
   }
 
@@ -250,24 +251,16 @@ export function verifyGeolocation(geolocation?: types.Geolocation) {
     return;
   geolocation.accuracy = geolocation.accuracy || 0;
   const { longitude, latitude, accuracy } = geolocation;
-  if (!helper.isNumber(longitude))
-    throw new Error(`geolocation.longitude: expected number, got ${typeof longitude}`);
   if (longitude < -180 || longitude > 180)
     throw new Error(`geolocation.longitude: precondition -180 <= LONGITUDE <= 180 failed.`);
-  if (!helper.isNumber(latitude))
-    throw new Error(`geolocation.latitude: expected number, got ${typeof latitude}`);
   if (latitude < -90 || latitude > 90)
     throw new Error(`geolocation.latitude: precondition -90 <= LATITUDE <= 90 failed.`);
-  if (!helper.isNumber(accuracy))
-    throw new Error(`geolocation.accuracy: expected number, got ${typeof accuracy}`);
   if (accuracy < 0)
     throw new Error(`geolocation.accuracy: precondition 0 <= ACCURACY failed.`);
 }
 
 export function verifyProxySettings(proxy: types.ProxySettings): types.ProxySettings {
   let { server, bypass } = proxy;
-  if (!helper.isString(server))
-    throw new Error(`Invalid proxy.server: ` + server);
   let url = new URL(server);
   if (!['http:', 'https:', 'socks5:'].includes(url.protocol)) {
     url = new URL('http://' + server);
diff --git a/src/chromium/crBrowser.ts b/src/chromium/crBrowser.ts
index be97bf7614..8496896b07 100644
--- a/src/chromium/crBrowser.ts
+++ b/src/chromium/crBrowser.ts
@@ -17,7 +17,7 @@
 
 import { Browser, BrowserOptions } from '../browser';
 import { assertBrowserContextIsNotOwned, BrowserContext, validateBrowserContextOptions, verifyGeolocation } from '../browserContext';
-import { assert } from '../helper';
+import { assert } from '../utils/utils';
 import * as network from '../network';
 import { Page, PageBinding, Worker } from '../page';
 import { ConnectionTransport } from '../transport';
diff --git a/src/chromium/crConnection.ts b/src/chromium/crConnection.ts
index 47acc41ee2..f0b5f1e24f 100644
--- a/src/chromium/crConnection.ts
+++ b/src/chromium/crConnection.ts
@@ -15,11 +15,12 @@
  * limitations under the License.
  */
 
-import { assert, debugLogger } from '../helper';
+import { assert } from '../utils/utils';
 import { ConnectionTransport, ProtocolRequest, ProtocolResponse } from '../transport';
 import { Protocol } from './protocol';
 import { EventEmitter } from 'events';
 import { rewriteErrorMessage } from '../utils/stackTrace';
+import { debugLogger } from '../utils/debugLogger';
 
 export const ConnectionEvents = {
   Disconnected: Symbol('ConnectionEvents.Disconnected')
diff --git a/src/chromium/crCoverage.ts b/src/chromium/crCoverage.ts
index 41c7ed980b..ceb24a58ec 100644
--- a/src/chromium/crCoverage.ts
+++ b/src/chromium/crCoverage.ts
@@ -16,10 +16,11 @@
  */
 
 import { CRSession } from './crConnection';
-import { assert, helper, RegisteredListener } from '../helper';
+import { helper, RegisteredListener } from '../helper';
 import { Protocol } from './protocol';
 import * as types from '../types';
 import * as sourceMap from '../utils/sourceMap';
+import { assert } from '../utils/utils';
 
 export class CRCoverage {
   private _jsCoverage: JSCoverage;
diff --git a/src/chromium/crInput.ts b/src/chromium/crInput.ts
index 15e30894a2..6b6edf9fe5 100644
--- a/src/chromium/crInput.ts
+++ b/src/chromium/crInput.ts
@@ -19,7 +19,7 @@ import * as input from '../input';
 import * as types from '../types';
 import { CRSession } from './crConnection';
 import { macEditingCommands } from '../macEditingCommands';
-import { helper } from '../helper';
+import { isString } from '../utils/utils';
 
 function toModifiersMask(modifiers: Set<types.KeyboardModifier>): number {
   let mask = 0;
@@ -51,7 +51,7 @@ export class RawKeyboardImpl implements input.RawKeyboard {
     parts.push(code);
     const shortcut = parts.join('+');
     let commands = macEditingCommands[shortcut] || [];
-    if (helper.isString(commands))
+    if (isString(commands))
       commands = [commands];
     // remove the trailing : to match the Chromium command names.
     return commands.map(c => c.substring(0, c.length - 1));
diff --git a/src/chromium/crNetworkManager.ts b/src/chromium/crNetworkManager.ts
index 6bed18c836..57c3044962 100644
--- a/src/chromium/crNetworkManager.ts
+++ b/src/chromium/crNetworkManager.ts
@@ -17,13 +17,13 @@
 
 import { CRSession } from './crConnection';
 import { Page } from '../page';
-import { assert, helper, RegisteredListener } from '../helper';
+import { helper, RegisteredListener } from '../helper';
 import { Protocol } from './protocol';
 import * as network from '../network';
 import * as frames from '../frames';
 import * as types from '../types';
 import { CRPage } from './crPage';
-import { headersObjectToArray } from '../converters';
+import { assert, headersObjectToArray } from '../utils/utils';
 
 export class CRNetworkManager {
   private _client: CRSession;
diff --git a/src/chromium/crPage.ts b/src/chromium/crPage.ts
index 0dbb46ffc9..c0240a48ad 100644
--- a/src/chromium/crPage.ts
+++ b/src/chromium/crPage.ts
@@ -17,7 +17,7 @@
 
 import * as dom from '../dom';
 import * as frames from '../frames';
-import { helper, RegisteredListener, assert } from '../helper';
+import { helper, RegisteredListener } from '../helper';
 import * as network from '../network';
 import { CRSession, CRConnection, CRSessionEvents } from './crConnection';
 import { CRExecutionContext } from './crExecutionContext';
@@ -36,7 +36,7 @@ import * as types from '../types';
 import { ConsoleMessage } from '../console';
 import * as sourceMap from '../utils/sourceMap';
 import { rewriteErrorMessage } from '../utils/stackTrace';
-import { headersArrayToObject } from '../converters';
+import { assert, headersArrayToObject } from '../utils/utils';
 
 
 const UTILITY_WORLD_NAME = '__playwright_utility_world__';
diff --git a/src/chromium/crPdf.ts b/src/chromium/crPdf.ts
index aa1f9adf6d..4f6d62136f 100644
--- a/src/chromium/crPdf.ts
+++ b/src/chromium/crPdf.ts
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-import { assert } from '../helper';
+import { assert } from '../utils/utils';
 import * as types from '../types';
 import { CRSession } from './crConnection';
 import { readProtocolStream } from './crProtocolHelper';
diff --git a/src/chromium/crProtocolHelper.ts b/src/chromium/crProtocolHelper.ts
index d433ff5108..5e8f31bf37 100644
--- a/src/chromium/crProtocolHelper.ts
+++ b/src/chromium/crProtocolHelper.ts
@@ -20,7 +20,7 @@ import { Protocol } from './protocol';
 import * as fs from 'fs';
 import * as util from 'util';
 import * as types from '../types';
-import { mkdirIfNeeded } from '../helper';
+import { mkdirIfNeeded } from '../utils/utils';
 
 export function getExceptionMessage(exceptionDetails: Protocol.Runtime.ExceptionDetails): string {
   if (exceptionDetails.exception)
diff --git a/src/converters.ts b/src/converters.ts
deleted file mode 100644
index 68d676b9c1..0000000000
--- a/src/converters.ts
+++ /dev/null
@@ -1,33 +0,0 @@
-/**
- * Copyright (c) Microsoft Corporation.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-import * as types from './types';
-
-export function headersObjectToArray(headers: { [key: string]: string }): types.HeadersArray {
-  const result: types.HeadersArray = [];
-  for (const name in headers) {
-    if (!Object.is(headers[name], undefined))
-      result.push({ name, value: headers[name] });
-  }
-  return result;
-}
-
-export function headersArrayToObject(headers: types.HeadersArray, lowerCase: boolean): { [key: string]: string } {
-  const result: { [key: string]: string } = {};
-  for (const { name, value } of headers)
-    result[lowerCase ? name.toLowerCase() : name] = value;
-  return result;
-}
diff --git a/src/dialog.ts b/src/dialog.ts
index 2fb35ede5e..1b38557eb2 100644
--- a/src/dialog.ts
+++ b/src/dialog.ts
@@ -15,7 +15,8 @@
  * limitations under the License.
  */
 
-import { assert, debugLogger } from './helper';
+import { assert } from './utils/utils';
+import { debugLogger } from './utils/debugLogger';
 
 type OnHandle = (accept: boolean, promptText?: string) => Promise<void>;
 
diff --git a/src/dom.ts b/src/dom.ts
index eb5677e846..e7bf07044d 100644
--- a/src/dom.ts
+++ b/src/dom.ts
@@ -15,7 +15,7 @@
  */
 
 import * as frames from './frames';
-import { assert, helper } from './helper';
+import { assert } from './utils/utils';
 import InjectedScript from './injected/injectedScript';
 import * as injectedScriptSource from './generated/injectedScriptSource';
 import * as debugScriptSource from './generated/debugScriptSource';
@@ -429,7 +429,6 @@ export class ElementHandle<T extends Node = Node> extends js.JSHandle<T> {
 
   async _fill(progress: Progress, value: string, options: types.NavigatingActionWaitOptions): Promise<'error:notconnected' | 'done'> {
     progress.log(`elementHandle.fill("${value}")`);
-    assert(helper.isString(value), `value: expected string, got ${typeof value}`);
     return this._page._frameManager.waitForSignalsCreatedBy(progress, options.noWaitAfter, async () => {
       progress.log('  waiting for element to be visible, enabled and editable');
       const poll = await this._evaluateHandleInUtility(([injected, node, value]) => {
diff --git a/src/download.ts b/src/download.ts
index 93e9e0c268..4cf3a1e699 100644
--- a/src/download.ts
+++ b/src/download.ts
@@ -19,7 +19,7 @@ import * as fs from 'fs';
 import * as util from 'util';
 import { Page } from './page';
 import { Readable } from 'stream';
-import { assert, mkdirIfNeeded } from './helper';
+import { assert, mkdirIfNeeded } from './utils/utils';
 
 export class Download {
   private _downloadsPath: string;
diff --git a/src/firefox/ffBrowser.ts b/src/firefox/ffBrowser.ts
index 6627724228..bb88fe0125 100644
--- a/src/firefox/ffBrowser.ts
+++ b/src/firefox/ffBrowser.ts
@@ -17,7 +17,8 @@
 
 import { Browser, BrowserOptions } from '../browser';
 import { assertBrowserContextIsNotOwned, BrowserContext, validateBrowserContextOptions, verifyGeolocation } from '../browserContext';
-import { assert, helper, RegisteredListener } from '../helper';
+import { helper, RegisteredListener } from '../helper';
+import { assert } from '../utils/utils';
 import * as network from '../network';
 import { Page, PageBinding } from '../page';
 import { ConnectionTransport } from '../transport';
diff --git a/src/firefox/ffConnection.ts b/src/firefox/ffConnection.ts
index 8c057bc05a..7c9c272e09 100644
--- a/src/firefox/ffConnection.ts
+++ b/src/firefox/ffConnection.ts
@@ -16,10 +16,11 @@
  */
 
 import { EventEmitter } from 'events';
-import { assert, debugLogger } from '../helper';
+import { assert } from '../utils/utils';
 import { ConnectionTransport, ProtocolRequest, ProtocolResponse } from '../transport';
 import { Protocol } from './protocol';
 import { rewriteErrorMessage } from '../utils/stackTrace';
+import { debugLogger } from '../utils/debugLogger';
 
 export const ConnectionEvents = {
   Disconnected: Symbol('Disconnected'),
diff --git a/src/firefox/ffPage.ts b/src/firefox/ffPage.ts
index 01353e87ef..89c37ff701 100644
--- a/src/firefox/ffPage.ts
+++ b/src/firefox/ffPage.ts
@@ -18,7 +18,8 @@
 import * as dialog from '../dialog';
 import * as dom from '../dom';
 import * as frames from '../frames';
-import { assert, helper, RegisteredListener } from '../helper';
+import { helper, RegisteredListener } from '../helper';
+import { assert } from '../utils/utils';
 import { Page, PageBinding, PageDelegate, Worker } from '../page';
 import { kScreenshotDuringNavigationError } from '../screenshotter';
 import * as types from '../types';
diff --git a/src/frames.ts b/src/frames.ts
index 79b99887a0..f4f94d8edd 100644
--- a/src/frames.ts
+++ b/src/frames.ts
@@ -17,7 +17,7 @@
 
 import { ConsoleMessage } from './console';
 import * as dom from './dom';
-import { assert, helper, RegisteredListener, debugLogger } from './helper';
+import { helper, RegisteredListener } from './helper';
 import * as js from './javascript';
 import * as network from './network';
 import { Page } from './page';
@@ -26,6 +26,8 @@ import * as types from './types';
 import { BrowserContext } from './browserContext';
 import { Progress, ProgressController } from './progress';
 import { EventEmitter } from 'events';
+import { assert, makeWaitForNextTask } from './utils/utils';
+import { debugLogger } from './utils/debugLogger';
 
 type ContextData = {
   contextPromise: Promise<dom.FrameExecutionContext>;
@@ -134,7 +136,7 @@ export class FrameManager {
     await barrier.waitFor();
     this._signalBarriers.delete(barrier);
     // Resolve in the next task, after all waitForNavigations.
-    await new Promise(helper.makeWaitForNextTask());
+    await new Promise(makeWaitForNextTask());
     return result;
   }
 
@@ -879,20 +881,15 @@ export class Frame extends EventEmitter {
   }
 
   async _waitForFunctionExpression<R>(expression: string, isFunction: boolean, arg: any, options: types.WaitForFunctionOptions = {}): Promise<js.SmartHandle<R>> {
-    const { polling = 'raf' } = options;
-    if (helper.isString(polling))
-      assert(polling === 'raf', 'Unknown polling option: ' + polling);
-    else if (helper.isNumber(polling))
-      assert(polling > 0, 'Cannot poll with non-positive interval: ' + polling);
-    else
-      throw new Error('Unknown polling option: ' + polling);
+    if (typeof options.pollingInterval === 'number')
+      assert(options.pollingInterval > 0, 'Cannot poll with non-positive interval: ' + options.pollingInterval);
     const predicateBody = isFunction ? 'return (' + expression + ')(arg)' :  'return (' + expression + ')';
     const task: dom.SchedulableTask<R> = injectedScript => injectedScript.evaluateHandle((injectedScript, { predicateBody, polling, arg }) => {
       const innerPredicate = new Function('arg', predicateBody) as (arg: any) => R;
-      if (polling === 'raf')
+      if (typeof polling !== 'number')
         return injectedScript.pollRaf((progress, continuePolling) => innerPredicate(arg) || continuePolling);
       return injectedScript.pollInterval(polling, (progress, continuePolling) => innerPredicate(arg) || continuePolling);
-    }, { predicateBody, polling, arg });
+    }, { predicateBody, polling: options.pollingInterval, arg });
     return this._page._runAbortableTask(
         progress => this._scheduleRerunnableHandleTask(progress, 'main', task),
         this._page._timeoutSettings.timeout(options));
diff --git a/src/helper.ts b/src/helper.ts
index ea534bee34..3e27e1ab84 100644
--- a/src/helper.ts
+++ b/src/helper.ts
@@ -21,14 +21,11 @@ import * as fs from 'fs';
 import * as os from 'os';
 import * as removeFolder from 'rimraf';
 import * as util from 'util';
-import * as path from 'path';
 import * as types from './types';
 import { Progress } from './progress';
-import * as debug from 'debug';
 
 const removeFolderAsync = util.promisify(removeFolder);
 const readFileAsync = util.promisify(fs.readFile.bind(fs));
-const mkdirAsync = util.promisify(fs.mkdir.bind(fs));
 
 export type RegisteredListener = {
   emitter: EventEmitter;
@@ -36,10 +33,6 @@ export type RegisteredListener = {
   handler: (...args: any[]) => void;
 };
 
-export type Listener = (...args: any[]) => void;
-
-const isInDebugMode = !!getFromENV('PWDEBUG');
-
 class Helper {
   static addEventListener(
     emitter: EventEmitter,
@@ -59,30 +52,6 @@ class Helper {
     listeners.splice(0, listeners.length);
   }
 
-  static isString(obj: any): obj is string {
-    return typeof obj === 'string' || obj instanceof String;
-  }
-
-  static isNumber(obj: any): obj is number {
-    return typeof obj === 'number' || obj instanceof Number;
-  }
-
-  static isRegExp(obj: any): obj is RegExp {
-    return obj instanceof RegExp || Object.prototype.toString.call(obj) === '[object RegExp]';
-  }
-
-  static isError(obj: any): obj is Error {
-    return obj instanceof Error || (obj && obj.__proto__ && obj.__proto__.name === 'Error');
-  }
-
-  static isObject(obj: any): obj is NonNullable<object> {
-    return typeof obj === 'object' && obj !== null;
-  }
-
-  static isBoolean(obj: any): obj is boolean {
-    return typeof obj === 'boolean' || obj instanceof Boolean;
-  }
-
   static completeUserURL(urlString: string): string {
     if (urlString.startsWith('localhost') || urlString.startsWith('127.0.0.1'))
       urlString = 'http://' + urlString;
@@ -101,41 +70,6 @@ class Helper {
     return { width: Math.floor(size.width + 1e-3), height: Math.floor(size.height + 1e-3) };
   }
 
-  // See https://joel.tools/microtasks/
-  static makeWaitForNextTask() {
-    if (parseInt(process.versions.node, 10) >= 11)
-      return setImmediate;
-
-    // Unlike Node 11, Node 10 and less have a bug with Task and MicroTask execution order:
-    // - https://github.com/nodejs/node/issues/22257
-    //
-    // So we can't simply run setImmediate to dispatch code in a following task.
-    // However, we can run setImmediate from-inside setImmediate to make sure we're getting
-    // in the following task.
-
-    let spinning = false;
-    const callbacks: (() => void)[] = [];
-    const loop = () => {
-      const callback = callbacks.shift();
-      if (!callback) {
-        spinning = false;
-        return;
-      }
-      setImmediate(loop);
-      // Make sure to call callback() as the last thing since it's
-      // untrusted code that might throw.
-      callback();
-    };
-
-    return (callback: () => void) => {
-      callbacks.push(callback);
-      if (!spinning) {
-        spinning = true;
-        setImmediate(loop);
-      }
-    };
-  }
-
   static guid(): string {
     return crypto.randomBytes(16).toString('hex');
   }
@@ -176,10 +110,6 @@ class Helper {
       progress.cleanupWhenAborted(dispose);
     return { promise, dispose };
   }
-
-  static isDebugMode(): boolean {
-    return isInDebugMode;
-  }
 }
 
 export async function getUbuntuVersion(): Promise<string> {
@@ -221,87 +151,4 @@ function getUbuntuVersionInternal(osReleaseText: string): string {
   return fields.get('version_id') || '';
 }
 
-export function assert(value: any, message?: string): asserts value {
-  if (!value)
-    throw new Error(message);
-}
-
-let _isUnderTest = false;
-
-export function setUnderTest() {
-  _isUnderTest = true;
-}
-
-export function isUnderTest(): boolean {
-  return _isUnderTest;
-}
-
-export function debugAssert(value: any, message?: string): asserts value {
-  if (_isUnderTest && !value)
-    throw new Error(message);
-}
-
-export function getFromENV(name: string) {
-  let value = process.env[name];
-  value = value || process.env[`npm_config_${name.toLowerCase()}`];
-  value = value || process.env[`npm_package_config_${name.toLowerCase()}`];
-  return value;
-}
-
-export async function doSlowMo(amount?: number) {
-  if (!amount)
-    return;
-  await new Promise(x => setTimeout(x, amount));
-}
-
-export async function mkdirIfNeeded(filePath: string) {
-  // This will harmlessly throw on windows if the dirname is the root directory.
-  await mkdirAsync(path.dirname(filePath), {recursive: true}).catch(() => {});
-}
-
 export const helper = Helper;
-
-const debugLoggerColorMap = {
-  'api': 45, // cyan
-  'protocol': 34, // green
-  'browser': 0, // reset
-  'error': 160, // red,
-  'channel:command': 33, // blue
-  'channel:response': 202, // orange
-  'channel:event': 207, // magenta
-};
-export type LogName = keyof typeof debugLoggerColorMap;
-
-export class DebugLogger {
-  private _debuggers = new Map<string, debug.IDebugger>();
-
-  constructor() {
-    if (process.env.DEBUG_FILE) {
-      const ansiRegex = new RegExp([
-        '[\\u001B\\u009B][[\\]()#;?]*(?:(?:(?:[a-zA-Z\\d]*(?:;[-a-zA-Z\\d\\/#&.:=?%@~_]*)*)?\\u0007)',
-        '(?:(?:\\d{1,4}(?:;\\d{0,4})*)?[\\dA-PR-TZcf-ntqry=><~]))'
-      ].join('|'), 'g');
-      const stream = fs.createWriteStream(process.env.DEBUG_FILE);
-      (debug as any).log = (data: string) => {
-        stream.write(data.replace(ansiRegex, ''));
-        stream.write('\n');
-      };
-    }
-  }
-
-  log(name: LogName, message: string | Error | object) {
-    let cachedDebugger = this._debuggers.get(name);
-    if (!cachedDebugger) {
-      cachedDebugger = debug(`pw:${name}`);
-      this._debuggers.set(name, cachedDebugger);
-      (cachedDebugger as any).color = debugLoggerColorMap[name];
-    }
-    cachedDebugger(message);
-  }
-
-  isEnabled(name: LogName) {
-    return debug.enabled(`pw:${name}`);
-  }
-}
-
-export const debugLogger = new DebugLogger();
diff --git a/src/input.ts b/src/input.ts
index 276aa2e2ab..39bf81e2a1 100644
--- a/src/input.ts
+++ b/src/input.ts
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { assert } from './helper';
+import { assert } from './utils/utils';
 import * as keyboardLayout from './usKeyboardLayout';
 import * as types from './types';
 import type { Page } from './page';
diff --git a/src/install/browserFetcher.ts b/src/install/browserFetcher.ts
index 4c5f6d7e41..67157c0997 100644
--- a/src/install/browserFetcher.ts
+++ b/src/install/browserFetcher.ts
@@ -23,7 +23,7 @@ import * as ProgressBar from 'progress';
 import { getProxyForUrl } from 'proxy-from-env';
 import * as URL from 'url';
 import * as util from 'util';
-import { assert, getFromENV } from '../helper';
+import { assert, getFromENV } from '../utils/utils';
 import * as browserPaths from './browserPaths';
 import { BrowserName, BrowserPlatform, BrowserDescriptor } from './browserPaths';
 
diff --git a/src/install/browserPaths.ts b/src/install/browserPaths.ts
index e302b111a7..1fb534f880 100644
--- a/src/install/browserPaths.ts
+++ b/src/install/browserPaths.ts
@@ -18,7 +18,8 @@
 import { execSync } from 'child_process';
 import * as os from 'os';
 import * as path from 'path';
-import { getFromENV, getUbuntuVersionSync } from '../helper';
+import { getUbuntuVersionSync } from '../helper';
+import { getFromENV } from '../utils/utils';
 
 export type BrowserName = 'chromium'|'webkit'|'firefox';
 export type BrowserPlatform = 'win32'|'win64'|'mac10.13'|'mac10.14'|'mac10.15'|'ubuntu18.04'|'ubuntu20.04';
diff --git a/src/install/installer.ts b/src/install/installer.ts
index c200b31e7f..0440a9cf16 100644
--- a/src/install/installer.ts
+++ b/src/install/installer.ts
@@ -15,13 +15,13 @@
  */
 
 import * as crypto from 'crypto';
-import { getFromENV } from '../helper';
 import * as fs from 'fs';
 import * as path from 'path';
 import * as util from 'util';
 import * as removeFolder from 'rimraf';
 import * as browserPaths from './browserPaths';
 import * as browserFetcher from './browserFetcher';
+import { getFromENV } from '../utils/utils';
 
 const fsMkdirAsync = util.promisify(fs.mkdir.bind(fs));
 const fsReaddirAsync = util.promisify(fs.readdir.bind(fs));
diff --git a/src/network.ts b/src/network.ts
index 93448aac39..974135a424 100644
--- a/src/network.ts
+++ b/src/network.ts
@@ -16,7 +16,7 @@
 
 import * as frames from './frames';
 import * as types from './types';
-import { assert } from './helper';
+import { assert } from './utils/utils';
 
 export function filterCookies(cookies: types.NetworkCookie[], urls: string[]): types.NetworkCookie[] {
   const parsedURLs = urls.map(s => new URL(s));
diff --git a/src/page.ts b/src/page.ts
index 5527530b2b..4085371ba7 100644
--- a/src/page.ts
+++ b/src/page.ts
@@ -17,12 +17,11 @@
 
 import * as dom from './dom';
 import * as frames from './frames';
-import { assert, helper, debugLogger } from './helper';
 import * as input from './input';
 import * as js from './javascript';
 import * as network from './network';
 import { Screenshotter } from './screenshotter';
-import { TimeoutSettings } from './timeoutSettings';
+import { TimeoutSettings } from './utils/timeoutSettings';
 import * as types from './types';
 import { BrowserContext } from './browserContext';
 import { ConsoleMessage } from './console';
@@ -30,6 +29,8 @@ import * as accessibility from './accessibility';
 import { EventEmitter } from 'events';
 import { FileChooser } from './fileChooser';
 import { Progress, runAbortableTask } from './progress';
+import { assert, isError } from './utils/utils';
+import { debugLogger } from './utils/debugLogger';
 
 export interface PageDelegate {
   readonly rawMouse: input.RawMouse;
@@ -462,7 +463,7 @@ export class PageBinding {
       const result = await binding!.playwrightFunction({ frame: context.frame, page, context: page._browserContext }, ...args);
       context.evaluateInternal(deliverResult, { name, seq, result }).catch(e => debugLogger.log('error', e));
     } catch (error) {
-      if (helper.isError(error))
+      if (isError(error))
         context.evaluateInternal(deliverError, { name, seq, message: error.message, stack: error.stack }).catch(e => debugLogger.log('error', e));
       else
         context.evaluateInternal(deliverErrorValue, { name, seq, error }).catch(e => debugLogger.log('error', e));
diff --git a/src/progress.ts b/src/progress.ts
index 436bc096e8..ce05769724 100644
--- a/src/progress.ts
+++ b/src/progress.ts
@@ -15,8 +15,9 @@
  */
 
 import { TimeoutError } from './utils/errors';
-import { assert, debugLogger, LogName } from './helper';
+import { assert } from './utils/utils';
 import { rewriteErrorMessage } from './utils/stackTrace';
+import { debugLogger, LogName } from './utils/debugLogger';
 
 export interface Progress {
   readonly aborted: Promise<void>;
diff --git a/src/protocol/serializers.ts b/src/protocol/serializers.ts
index aee1c99c9e..7e404c65b1 100644
--- a/src/protocol/serializers.ts
+++ b/src/protocol/serializers.ts
@@ -15,11 +15,10 @@
  */
 
 import { TimeoutError } from '../utils/errors';
-import { helper } from '../helper';
 import { SerializedError, SerializedValue } from './channels';
 
 export function serializeError(e: any): SerializedError {
-  if (helper.isError(e))
+  if (isError(e))
     return { error: { message: e.message, stack: e.stack, name: e.name } };
   return { value: serializeValue(e, value => ({ fallThrough: value }), new Set()) };
 }
diff --git a/src/protocol/transport.ts b/src/protocol/transport.ts
index 6234d32abe..fbd2fabcc5 100644
--- a/src/protocol/transport.ts
+++ b/src/protocol/transport.ts
@@ -14,12 +14,12 @@
  * limitations under the License.
  */
 
-import { helper } from '../helper';
+import { makeWaitForNextTask } from '../utils/utils';
 
 export class Transport {
   private _pipeWrite: NodeJS.WritableStream;
   private _data = Buffer.from([]);
-  private _waitForNextTask = helper.makeWaitForNextTask();
+  private _waitForNextTask = makeWaitForNextTask();
   private _closed = false;
   private _bytesLeft = 0;
 
diff --git a/src/protocol/validatorPrimitives.ts b/src/protocol/validatorPrimitives.ts
index 215cb6e976..ac996b078d 100644
--- a/src/protocol/validatorPrimitives.ts
+++ b/src/protocol/validatorPrimitives.ts
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { isUnderTest } from '../helper';
+import { isUnderTest } from '../utils/utils';
 
 export class ValidationError extends Error {}
 export type Validator = (arg: any, path: string) => any;
diff --git a/src/rpc/client/browser.ts b/src/rpc/client/browser.ts
index 912e2b7bac..72175d9776 100644
--- a/src/rpc/client/browser.ts
+++ b/src/rpc/client/browser.ts
@@ -20,9 +20,9 @@ import { Page } from './page';
 import { ChannelOwner } from './channelOwner';
 import { Events } from './events';
 import { BrowserType } from './browserType';
-import { headersObjectToArray } from '../../converters';
 import { BrowserContextOptions } from './types';
 import { validateHeaders } from './network';
+import { headersObjectToArray } from '../../utils/utils';
 
 export class Browser extends ChannelOwner<BrowserChannel, BrowserInitializer> {
   readonly _contexts = new Set<BrowserContext>();
diff --git a/src/rpc/client/browserContext.ts b/src/rpc/client/browserContext.ts
index 7b48f8017b..8dd4028e7e 100644
--- a/src/rpc/client/browserContext.ts
+++ b/src/rpc/client/browserContext.ts
@@ -20,13 +20,13 @@ import { Page, BindingCall } from './page';
 import * as network from './network';
 import { BrowserContextChannel, BrowserContextInitializer } from '../../protocol/channels';
 import { ChannelOwner } from './channelOwner';
-import { isUnderTest, deprecate, evaluationScript, urlMatches } from './clientHelper';
+import { deprecate, evaluationScript, urlMatches } from './clientHelper';
 import { Browser } from './browser';
 import { Events } from './events';
-import { TimeoutSettings } from '../../timeoutSettings';
+import { TimeoutSettings } from '../../utils/timeoutSettings';
 import { Waiter } from './waiter';
-import { headersObjectToArray } from '../../converters';
 import { URLMatch, Headers, WaitForEventOptions } from './types';
+import { isUnderTest, headersObjectToArray } from '../../utils/utils';
 
 export class BrowserContext extends ChannelOwner<BrowserContextChannel, BrowserContextInitializer> {
   _pages = new Set<Page>();
diff --git a/src/rpc/client/browserType.ts b/src/rpc/client/browserType.ts
index 8675e73ce1..69e51956ef 100644
--- a/src/rpc/client/browserType.ts
+++ b/src/rpc/client/browserType.ts
@@ -18,17 +18,16 @@ import { BrowserTypeChannel, BrowserTypeInitializer, BrowserTypeLaunchParams, Br
 import { Browser } from './browser';
 import { BrowserContext } from './browserContext';
 import { ChannelOwner } from './channelOwner';
-import { headersObjectToArray } from '../../converters';
-import { assert, helper } from '../../helper';
 import { LaunchOptions, LaunchServerOptions, ConnectOptions, LaunchPersistentContextOptions } from './types';
 import * as WebSocket from 'ws';
 import { Connection } from './connection';
 import { serializeError } from '../../protocol/serializers';
 import { Events } from './events';
-import { TimeoutSettings } from '../../timeoutSettings';
+import { TimeoutSettings } from '../../utils/timeoutSettings';
 import { ChildProcess } from 'child_process';
 import { envObjectToArray } from './clientHelper';
 import { validateHeaders } from './network';
+import { assert, makeWaitForNextTask, headersObjectToArray } from '../../utils/utils';
 
 export interface BrowserServerLauncher {
   launchServer(options?: LaunchServerOptions): Promise<BrowserServer>;
@@ -122,7 +121,7 @@ export class BrowserType extends ChannelOwner<BrowserTypeChannel, BrowserTypeIni
       // The 'ws' module in node sometimes sends us multiple messages in a single task.
       const waitForNextTask = options.slowMo
         ? (cb: () => any) => setTimeout(cb, options.slowMo)
-        : helper.makeWaitForNextTask();
+        : makeWaitForNextTask();
       connection.onmessage = message => {
         if (ws.readyState !== WebSocket.OPEN) {
           setTimeout(() => {
diff --git a/src/rpc/client/channelOwner.ts b/src/rpc/client/channelOwner.ts
index 89251f5772..44fbceec96 100644
--- a/src/rpc/client/channelOwner.ts
+++ b/src/rpc/client/channelOwner.ts
@@ -18,7 +18,7 @@ import { EventEmitter } from 'events';
 import type { Channel } from '../../protocol/channels';
 import type { Connection } from './connection';
 import type { LoggerSink } from './types';
-import { debugLogger } from '../../helper';
+import { debugLogger } from '../../utils/debugLogger';
 
 export abstract class ChannelOwner<T extends Channel = Channel, Initializer = {}> extends EventEmitter {
   private _connection: Connection;
diff --git a/src/rpc/client/clientHelper.ts b/src/rpc/client/clientHelper.ts
index 53881ada27..cf13c9201d 100644
--- a/src/rpc/client/clientHelper.ts
+++ b/src/rpc/client/clientHelper.ts
@@ -15,10 +15,10 @@
  * limitations under the License.
  */
 
-import { isUnderTest as commonIsUnderTest, helper } from '../../helper';
 import * as types from './types';
 import * as fs from 'fs';
 import * as util from 'util';
+import { isString, isRegExp } from '../../utils/utils';
 
 const deprecatedHits = new Set();
 export function deprecate(methodName: string, message: string) {
@@ -28,10 +28,6 @@ export function deprecate(methodName: string, message: string) {
   console.warn(message);
 }
 
-export function isUnderTest() {
-  return commonIsUnderTest();
-}
-
 export function envObjectToArray(env: types.Env): { name: string, value: string }[] {
   const result: { name: string, value: string }[] = [];
   for (const name in env) {
@@ -49,7 +45,7 @@ export async function evaluationScript(fun: Function | string | { path?: string,
   }
   if (arg !== undefined)
     throw new Error('Cannot evaluate a string with arguments');
-  if (helper.isString(fun))
+  if (isString(fun))
     return fun;
   if (fun.content !== undefined)
     return fun.content;
@@ -65,9 +61,9 @@ export async function evaluationScript(fun: Function | string | { path?: string,
 export function urlMatches(urlString: string, match: types.URLMatch | undefined): boolean {
   if (match === undefined || match === '')
     return true;
-  if (helper.isString(match))
+  if (isString(match))
     match = globToRegex(match);
-  if (helper.isRegExp(match))
+  if (isRegExp(match))
     return match.test(urlString);
   if (typeof match === 'string' && match === urlString)
     return true;
diff --git a/src/rpc/client/connection.ts b/src/rpc/client/connection.ts
index b55d2f67de..49990bfe45 100644
--- a/src/rpc/client/connection.ts
+++ b/src/rpc/client/connection.ts
@@ -39,7 +39,7 @@ import { Stream } from './stream';
 import { createScheme, Validator, ValidationError } from '../../protocol/validator';
 import { WebKitBrowser } from './webkitBrowser';
 import { FirefoxBrowser } from './firefoxBrowser';
-import { debugLogger } from '../../helper';
+import { debugLogger } from '../../utils/debugLogger';
 
 class Root extends ChannelOwner<Channel, {}> {
   constructor(connection: Connection) {
diff --git a/src/rpc/client/electron.ts b/src/rpc/client/electron.ts
index 6ecedd309a..92704276ee 100644
--- a/src/rpc/client/electron.ts
+++ b/src/rpc/client/electron.ts
@@ -19,7 +19,7 @@ import { BrowserContext } from './browserContext';
 import { ChannelOwner } from './channelOwner';
 import { Page } from './page';
 import { serializeArgument, FuncOn, parseResult, SmartHandle, JSHandle } from './jsHandle';
-import { TimeoutSettings } from '../../timeoutSettings';
+import { TimeoutSettings } from '../../utils/timeoutSettings';
 import { Waiter } from './waiter';
 import { Events } from './events';
 import { WaitForEventOptions, Env, LoggerSink } from './types';
diff --git a/src/rpc/client/elementHandle.ts b/src/rpc/client/elementHandle.ts
index 7ceabb7236..88181fa9db 100644
--- a/src/rpc/client/elementHandle.ts
+++ b/src/rpc/client/elementHandle.ts
@@ -18,12 +18,12 @@ import { ElementHandleChannel, JSHandleInitializer, ElementHandleScrollIntoViewI
 import { Frame } from './frame';
 import { FuncOn, JSHandle, serializeArgument, parseResult } from './jsHandle';
 import { ChannelOwner } from './channelOwner';
-import { helper, assert, mkdirIfNeeded } from '../../helper';
 import { SelectOption, FilePayload, Rect, SelectOptionOptions } from './types';
 import * as fs from 'fs';
 import * as mime from 'mime';
 import * as path from 'path';
 import * as util from 'util';
+import { assert, isString, mkdirIfNeeded } from '../../utils/utils';
 
 const fsWriteFileAsync = util.promisify(fs.writeFile.bind(fs));
 
@@ -246,7 +246,7 @@ export function convertSelectOptionValues(values: string | ElementHandle | Selec
     assert(values[i] !== null, `options[${i}]: expected object, got null`);
   if (values[0] instanceof ElementHandle)
     return { elements: (values as ElementHandle[]).map((v: ElementHandle) => v._elementChannel) };
-  if (helper.isString(values[0]))
+  if (isString(values[0]))
     return { options: (values as string[]).map(value => ({ value })) };
   return { options: values as SelectOption[] };
 }
diff --git a/src/rpc/client/frame.ts b/src/rpc/client/frame.ts
index ed8d51ba07..d96ceef22a 100644
--- a/src/rpc/client/frame.ts
+++ b/src/rpc/client/frame.ts
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-import { assert } from '../../helper';
+import { assert } from '../../utils/utils';
 import { FrameChannel, FrameInitializer, FrameNavigatedEvent, FrameGotoOptions, FrameWaitForSelectorOptions, FrameDispatchEventOptions, FrameSetContentOptions, FrameClickOptions, FrameDblclickOptions, FrameFillOptions, FrameFocusOptions, FrameTextContentOptions, FrameInnerTextOptions, FrameInnerHTMLOptions, FrameGetAttributeOptions, FrameHoverOptions, FrameSetInputFilesOptions, FrameTypeOptions, FramePressOptions, FrameCheckOptions, FrameUncheckOptions } from '../../protocol/channels';
 import { BrowserContext } from './browserContext';
 import { ChannelOwner } from './channelOwner';
diff --git a/src/rpc/client/network.ts b/src/rpc/client/network.ts
index 1d95bf8fa4..32cce3ee6f 100644
--- a/src/rpc/client/network.ts
+++ b/src/rpc/client/network.ts
@@ -18,12 +18,11 @@ import { URLSearchParams } from 'url';
 import { RequestChannel, ResponseChannel, RouteChannel, RequestInitializer, ResponseInitializer, RouteInitializer } from '../../protocol/channels';
 import { ChannelOwner } from './channelOwner';
 import { Frame } from './frame';
-import { headersArrayToObject, headersObjectToArray } from '../../converters';
 import { Headers } from './types';
 import * as fs from 'fs';
 import * as mime from 'mime';
 import * as util from 'util';
-import { helper } from '../../helper';
+import { isString, headersObjectToArray, headersArrayToObject } from '../../utils/utils';
 
 export type NetworkCookie = {
   name: string,
@@ -175,7 +174,7 @@ export class Route extends ChannelOwner<RouteChannel, RouteInitializer> {
       body = buffer.toString('base64');
       isBase64 = true;
       length = buffer.length;
-    } else if (helper.isString(response.body)) {
+    } else if (isString(response.body)) {
       body = response.body;
       isBase64 = false;
       length = Buffer.byteLength(body);
@@ -204,7 +203,7 @@ export class Route extends ChannelOwner<RouteChannel, RouteInitializer> {
   }
 
   async continue(overrides: { method?: string, headers?: Headers, postData?: string | Buffer } = {}) {
-    const postDataBuffer = helper.isString(overrides.postData) ? Buffer.from(overrides.postData, 'utf8') : overrides.postData;
+    const postDataBuffer = isString(overrides.postData) ? Buffer.from(overrides.postData, 'utf8') : overrides.postData;
     await this._channel.continue({
       method: overrides.method,
       headers: overrides.headers ? headersObjectToArray(overrides.headers) : undefined,
@@ -284,7 +283,7 @@ export class Response extends ChannelOwner<ResponseChannel, ResponseInitializer>
 export function validateHeaders(headers: Headers) {
   for (const key of Object.keys(headers)) {
     const value = headers[key];
-    if (!Object.is(value, undefined) && !helper.isString(value))
+    if (!Object.is(value, undefined) && !isString(value))
       throw new Error(`Expected value of header "${key}" to be String, but "${typeof value}" is found.`);
   }
 }
diff --git a/src/rpc/client/page.ts b/src/rpc/client/page.ts
index 9ba7c6bb3b..2ec897760e 100644
--- a/src/rpc/client/page.ts
+++ b/src/rpc/client/page.ts
@@ -16,11 +16,10 @@
  */
 
 import { Events } from './events';
-import { assert, helper, Listener, mkdirIfNeeded } from '../../helper';
-import { TimeoutSettings } from '../../timeoutSettings';
+import { assert } from '../../utils/utils';
+import { TimeoutSettings } from '../../utils/timeoutSettings';
 import { BindingCallChannel, BindingCallInitializer, PageChannel, PageInitializer, PagePdfParams, FrameWaitForSelectorOptions, FrameDispatchEventOptions, FrameSetContentOptions, FrameGotoOptions, PageReloadOptions, PageGoBackOptions, PageGoForwardOptions, PageScreenshotOptions, FrameClickOptions, FrameDblclickOptions, FrameFillOptions, FrameFocusOptions, FrameTextContentOptions, FrameInnerTextOptions, FrameInnerHTMLOptions, FrameGetAttributeOptions, FrameHoverOptions, FrameSetInputFilesOptions, FrameTypeOptions, FramePressOptions, FrameCheckOptions, FrameUncheckOptions } from '../../protocol/channels';
 import { parseError, serializeError } from '../../protocol/serializers';
-import { headersObjectToArray } from '../../converters';
 import { Accessibility } from './accessibility';
 import { BrowserContext } from './browserContext';
 import { ChannelOwner } from './channelOwner';
@@ -42,6 +41,7 @@ import * as fs from 'fs';
 import * as util from 'util';
 import { Size, URLMatch, Headers, LifecycleEvent, WaitForEventOptions, SelectOption, SelectOptionOptions, FilePayload, WaitForFunctionOptions } from './types';
 import { evaluationScript, urlMatches } from './clientHelper';
+import { isString, isRegExp, isObject, mkdirIfNeeded, headersObjectToArray } from '../../utils/utils';
 
 type PDFOptions = Omit<PagePdfParams, 'width' | 'height' | 'margin'> & {
   width?: string | number,
@@ -54,6 +54,7 @@ type PDFOptions = Omit<PagePdfParams, 'width' | 'height' | 'margin'> & {
   },
   path?: string,
 };
+type Listener = (...args: any[]) => void;
 
 const fsWriteFileAsync = util.promisify(fs.writeFile.bind(fs));
 
@@ -197,8 +198,8 @@ export class Page extends ChannelOwner<PageChannel, PageInitializer> {
   }
 
   frame(options: string | { name?: string, url?: URLMatch }): Frame | null {
-    const name = helper.isString(options) ? options : options.name;
-    const url = helper.isObject(options) ? options.url : undefined;
+    const name = isString(options) ? options : options.name;
+    const url = isObject(options) ? options.url : undefined;
     assert(name || url, 'Either name or url matcher should be specified');
     return this.frames().find(f => {
       if (name)
@@ -330,7 +331,7 @@ export class Page extends ChannelOwner<PageChannel, PageInitializer> {
 
   async waitForRequest(urlOrPredicate: string | RegExp | ((r: Request) => boolean), options: { timeout?: number } = {}): Promise<Request> {
     const predicate = (request: Request) => {
-      if (helper.isString(urlOrPredicate) || helper.isRegExp(urlOrPredicate))
+      if (isString(urlOrPredicate) || isRegExp(urlOrPredicate))
         return urlMatches(request.url(), urlOrPredicate);
       return urlOrPredicate(request);
     };
@@ -339,7 +340,7 @@ export class Page extends ChannelOwner<PageChannel, PageInitializer> {
 
   async waitForResponse(urlOrPredicate: string | RegExp | ((r: Response) => boolean), options: { timeout?: number } = {}): Promise<Response> {
     const predicate = (response: Response) => {
-      if (helper.isString(urlOrPredicate) || helper.isRegExp(urlOrPredicate))
+      if (isString(urlOrPredicate) || isRegExp(urlOrPredicate))
         return urlMatches(response.url(), urlOrPredicate);
       return urlOrPredicate(response);
     };
diff --git a/src/rpc/inprocess.ts b/src/rpc/inprocess.ts
index 017b55a7bb..b12174c8a7 100644
--- a/src/rpc/inprocess.ts
+++ b/src/rpc/inprocess.ts
@@ -19,8 +19,8 @@ import type { Playwright as PlaywrightImpl } from '../server/playwright';
 import type { Playwright as PlaywrightAPI } from './client/playwright';
 import { PlaywrightDispatcher } from './server/playwrightDispatcher';
 import { Connection } from './client/connection';
-import { isUnderTest } from '../helper';
 import { BrowserServerLauncherImpl } from './browserServerImpl';
+import { isUnderTest } from '../utils/utils';
 
 export function setupInProcess(playwright: PlaywrightImpl): PlaywrightAPI {
   const clientConnection = new Connection();
diff --git a/src/rpc/server/dispatcher.ts b/src/rpc/server/dispatcher.ts
index 13cb007e1d..7ccc00a9d3 100644
--- a/src/rpc/server/dispatcher.ts
+++ b/src/rpc/server/dispatcher.ts
@@ -15,10 +15,11 @@
  */
 
 import { EventEmitter } from 'events';
-import { helper, debugAssert, assert } from '../../helper';
+import { helper } from '../../helper';
 import * as channels from '../../protocol/channels';
 import { serializeError } from '../../protocol/serializers';
 import { createScheme, Validator, ValidationError } from '../../protocol/validator';
+import { assert, debugAssert } from '../../utils/utils';
 
 export const dispatcherSymbol = Symbol('dispatcher');
 
diff --git a/src/rpc/server/frameDispatcher.ts b/src/rpc/server/frameDispatcher.ts
index 6296f0ce4b..96a985bd44 100644
--- a/src/rpc/server/frameDispatcher.ts
+++ b/src/rpc/server/frameDispatcher.ts
@@ -173,11 +173,7 @@ export class FrameDispatcher extends Dispatcher<Frame, channels.FrameInitializer
   }
 
   async waitForFunction(params: channels.FrameWaitForFunctionParams): Promise<channels.FrameWaitForFunctionResult> {
-    const options = {
-      ...params,
-      polling: params.pollingInterval === undefined ? 'raf' as const : params.pollingInterval
-    };
-    return { handle: createHandle(this._scope, await this._frame._waitForFunctionExpression(params.expression, params.isFunction, parseArgument(params.arg), options)) };
+    return { handle: createHandle(this._scope, await this._frame._waitForFunctionExpression(params.expression, params.isFunction, parseArgument(params.arg), params)) };
   }
 
   async title(): Promise<channels.FrameTitleResult> {
diff --git a/src/screenshotter.ts b/src/screenshotter.ts
index fd33d8a33b..758883d1e4 100644
--- a/src/screenshotter.ts
+++ b/src/screenshotter.ts
@@ -16,11 +16,12 @@
  */
 
 import * as dom from './dom';
-import { assert, helper } from './helper';
+import { helper } from './helper';
 import { Page } from './page';
 import * as types from './types';
 import { rewriteErrorMessage } from './utils/stackTrace';
 import { Progress } from './progress';
+import { assert } from './utils/utils';
 
 export class Screenshotter {
   private _queue = new TaskQueue();
diff --git a/src/selectors.ts b/src/selectors.ts
index df40295c0a..dcc3820919 100644
--- a/src/selectors.ts
+++ b/src/selectors.ts
@@ -16,7 +16,6 @@
 
 import * as dom from './dom';
 import * as frames from './frames';
-import { helper, assert } from './helper';
 import * as js from './javascript';
 import * as types from './types';
 import { ParsedSelector, parseSelector } from './common/selectorParser';
@@ -116,7 +115,6 @@ export class Selectors {
   }
 
   _parseSelector(selector: string): SelectorInfo {
-    assert(helper.isString(selector), `selector must be a string`);
     const parsed = parseSelector(selector);
     for (const {name} of parsed.parts) {
       if (!this._builtinEngines.has(name) && !this._engines.has(name))
diff --git a/src/server/browserType.ts b/src/server/browserType.ts
index 20f1a1d747..a9b8c3edf8 100644
--- a/src/server/browserType.ts
+++ b/src/server/browserType.ts
@@ -22,13 +22,13 @@ import { BrowserContext, verifyProxySettings, validateBrowserContextOptions } fr
 import * as browserPaths from '../install/browserPaths';
 import { ConnectionTransport, WebSocketTransport } from '../transport';
 import { BrowserOptions, Browser, BrowserProcess } from '../browser';
-import { assert, helper } from '../helper';
 import { launchProcess, Env, waitForLine, envArrayToObject } from './processLauncher';
 import { PipeTransport } from './pipeTransport';
 import { Progress, runAbortableTask } from '../progress';
 import * as types from '../types';
-import { TimeoutSettings } from '../timeoutSettings';
+import { TimeoutSettings } from '../utils/timeoutSettings';
 import { validateHostRequirements } from './validateDependencies';
+import { assert, isDebugMode } from '../utils/utils';
 
 export interface BrowserType {
   executablePath(): string;
@@ -223,7 +223,7 @@ function copyTestHooks(from: object, to: object) {
 }
 
 function validateLaunchOptions<Options extends types.LaunchOptions>(options: Options): Options {
-  const { devtools = false, headless = !helper.isDebugMode() && !devtools } = options;
+  const { devtools = false, headless = !isDebugMode() && !devtools } = options;
   return { ...options, devtools, headless };
 }
 
diff --git a/src/server/chromium.ts b/src/server/chromium.ts
index 307e627496..f2a9ed3ec8 100644
--- a/src/server/chromium.ts
+++ b/src/server/chromium.ts
@@ -17,7 +17,6 @@
 
 import * as path from 'path';
 import * as os from 'os';
-import { getFromENV, helper } from '../helper';
 import { CRBrowser } from '../chromium/crBrowser';
 import { Env } from './processLauncher';
 import { kBrowserCloseMessageId } from '../chromium/crConnection';
@@ -28,6 +27,7 @@ import { BrowserDescriptor } from '../install/browserPaths';
 import { CRDevTools } from '../chromium/crDevTools';
 import { BrowserOptions } from '../browser';
 import * as types from '../types';
+import { isDebugMode, getFromENV } from '../utils/utils';
 
 export class Chromium extends BrowserTypeBase {
   private _devtools: CRDevTools | undefined;
@@ -43,7 +43,7 @@ export class Chromium extends BrowserTypeBase {
 
     super(packagePath, browser, debugPort ? { webSocketRegex: /^DevTools listening on (ws:\/\/.*)$/, stream: 'stderr' } : null);
     this._debugPort = debugPort;
-    if (helper.isDebugMode())
+    if (isDebugMode())
       this._devtools = this._createDevTools();
   }
 
diff --git a/src/server/electron.ts b/src/server/electron.ts
index acad60e540..d30dee1c34 100644
--- a/src/server/electron.ts
+++ b/src/server/electron.ts
@@ -20,7 +20,7 @@ import { CRConnection, CRSession } from '../chromium/crConnection';
 import { CRExecutionContext } from '../chromium/crExecutionContext';
 import * as js from '../javascript';
 import { Page } from '../page';
-import { TimeoutSettings } from '../timeoutSettings';
+import { TimeoutSettings } from '../utils/timeoutSettings';
 import { WebSocketTransport } from '../transport';
 import * as types from '../types';
 import { launchProcess, waitForLine, envArrayToObject } from './processLauncher';
diff --git a/src/server/pipeTransport.ts b/src/server/pipeTransport.ts
index 0e6a0a546f..fd8d08fa80 100644
--- a/src/server/pipeTransport.ts
+++ b/src/server/pipeTransport.ts
@@ -15,14 +15,16 @@
  * limitations under the License.
  */
 
-import { helper, RegisteredListener, debugLogger } from '../helper';
+import { helper, RegisteredListener } from '../helper';
 import { ConnectionTransport, ProtocolRequest, ProtocolResponse } from '../transport';
+import { makeWaitForNextTask } from '../utils/utils';
+import { debugLogger } from '../utils/debugLogger';
 
 export class PipeTransport implements ConnectionTransport {
   private _pipeWrite: NodeJS.WritableStream;
   private _pendingMessage = '';
   private _eventListeners: RegisteredListener[];
-  private _waitForNextTask = helper.makeWaitForNextTask();
+  private _waitForNextTask = makeWaitForNextTask();
   private _closed = false;
 
   onmessage?: (message: ProtocolResponse) => void;
diff --git a/src/server/processLauncher.ts b/src/server/processLauncher.ts
index 436cdeffa0..bfe6893723 100644
--- a/src/server/processLauncher.ts
+++ b/src/server/processLauncher.ts
@@ -19,9 +19,10 @@ import * as childProcess from 'child_process';
 import * as readline from 'readline';
 import * as removeFolder from 'rimraf';
 import * as stream from 'stream';
-import { helper, isUnderTest } from '../helper';
+import { helper } from '../helper';
 import { Progress } from '../progress';
 import * as types from '../types';
+import { isUnderTest } from '../utils/utils';
 
 export type Env = {[key: string]: string | number | boolean | undefined};
 
diff --git a/src/transport.ts b/src/transport.ts
index 3a3318bd06..1be82072ef 100644
--- a/src/transport.ts
+++ b/src/transport.ts
@@ -16,8 +16,8 @@
  */
 
 import * as WebSocket from 'ws';
-import { helper } from './helper';
 import { Progress } from './progress';
+import { makeWaitForNextTask } from './utils/utils';
 
 export type ProtocolRequest = {
   id: number;
@@ -85,7 +85,7 @@ export class WebSocketTransport implements ConnectionTransport {
     // In Web, all IO callbacks (e.g. WebSocket callbacks)
     // are dispatched into separate tasks, so there's no need
     // to do anything extra.
-    const messageWrap: (cb: () => void) => void = helper.makeWaitForNextTask();
+    const messageWrap: (cb: () => void) => void = makeWaitForNextTask();
 
     this._ws.addEventListener('message', event => {
       messageWrap(() => {
diff --git a/src/types.ts b/src/types.ts
index 8cfa88205e..9227154b1d 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -26,8 +26,7 @@ export type TimeoutOptions = { timeout?: number };
 
 export type WaitForElementOptions = TimeoutOptions & { state?: 'attached' | 'detached' | 'visible' | 'hidden' };
 
-export type Polling = 'raf' | number;
-export type WaitForFunctionOptions = TimeoutOptions & { polling?: Polling };
+export type WaitForFunctionOptions = TimeoutOptions & { pollingInterval?: number };
 
 export type LifecycleEvent = 'load' | 'domcontentloaded' | 'networkidle';
 export const kLifecycleEvents: Set<LifecycleEvent> = new Set(['load', 'domcontentloaded', 'networkidle']);
diff --git a/src/utils/debugLogger.ts b/src/utils/debugLogger.ts
new file mode 100644
index 0000000000..cbdf95dd0d
--- /dev/null
+++ b/src/utils/debugLogger.ts
@@ -0,0 +1,63 @@
+/**
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as debug from 'debug';
+import * as fs from 'fs';
+
+const debugLoggerColorMap = {
+  'api': 45, // cyan
+  'protocol': 34, // green
+  'browser': 0, // reset
+  'error': 160, // red,
+  'channel:command': 33, // blue
+  'channel:response': 202, // orange
+  'channel:event': 207, // magenta
+};
+export type LogName = keyof typeof debugLoggerColorMap;
+
+class DebugLogger {
+  private _debuggers = new Map<string, debug.IDebugger>();
+
+  constructor() {
+    if (process.env.DEBUG_FILE) {
+      const ansiRegex = new RegExp([
+        '[\\u001B\\u009B][[\\]()#;?]*(?:(?:(?:[a-zA-Z\\d]*(?:;[-a-zA-Z\\d\\/#&.:=?%@~_]*)*)?\\u0007)',
+        '(?:(?:\\d{1,4}(?:;\\d{0,4})*)?[\\dA-PR-TZcf-ntqry=><~]))'
+      ].join('|'), 'g');
+      const stream = fs.createWriteStream(process.env.DEBUG_FILE);
+      (debug as any).log = (data: string) => {
+        stream.write(data.replace(ansiRegex, ''));
+        stream.write('\n');
+      };
+    }
+  }
+
+  log(name: LogName, message: string | Error | object) {
+    let cachedDebugger = this._debuggers.get(name);
+    if (!cachedDebugger) {
+      cachedDebugger = debug(`pw:${name}`);
+      this._debuggers.set(name, cachedDebugger);
+      (cachedDebugger as any).color = debugLoggerColorMap[name];
+    }
+    cachedDebugger(message);
+  }
+
+  isEnabled(name: LogName) {
+    return debug.enabled(`pw:${name}`);
+  }
+}
+
+export const debugLogger = new DebugLogger();
diff --git a/src/utils/sourceMap.ts b/src/utils/sourceMap.ts
index 163ed8a67f..10a55b8b91 100644
--- a/src/utils/sourceMap.ts
+++ b/src/utils/sourceMap.ts
@@ -17,7 +17,7 @@
 import * as fs from 'fs';
 import * as util from 'util';
 import { getCallerFilePath } from './stackTrace';
-import { helper } from '../helper';
+import { isDebugMode } from './utils';
 
 type Position = {
   line: number;
@@ -37,7 +37,7 @@ export function ensureSourceUrl(expression: string): string {
 }
 
 export async function generateSourceMapUrl(functionText: string, generatedText: string): Promise<string> {
-  if (!helper.isDebugMode())
+  if (!isDebugMode())
     return generateSourceUrl();
   const sourceMapUrl = await innerGenerateSourceMapUrl(functionText, generatedText);
   return sourceMapUrl || generateSourceUrl();
diff --git a/src/timeoutSettings.ts b/src/utils/timeoutSettings.ts
similarity index 86%
rename from src/timeoutSettings.ts
rename to src/utils/timeoutSettings.ts
index ce318f61df..3188bd0267 100644
--- a/src/timeoutSettings.ts
+++ b/src/utils/timeoutSettings.ts
@@ -15,10 +15,9 @@
  * limitations under the License.
  */
 
-import { TimeoutOptions } from './types';
-import { helper } from './helper';
+import { isDebugMode } from './utils';
 
-const DEFAULT_TIMEOUT = helper.isDebugMode() ? 0 : 30000;
+const DEFAULT_TIMEOUT = isDebugMode() ? 0 : 30000;
 
 export class TimeoutSettings {
   private _parent: TimeoutSettings | undefined;
@@ -37,7 +36,7 @@ export class TimeoutSettings {
     this._defaultNavigationTimeout = timeout;
   }
 
-  navigationTimeout(options: TimeoutOptions): number {
+  navigationTimeout(options: { timeout?: number }): number {
     if (typeof options.timeout === 'number')
       return options.timeout;
     if (this._defaultNavigationTimeout !== null)
@@ -49,7 +48,7 @@ export class TimeoutSettings {
     return DEFAULT_TIMEOUT;
   }
 
-  timeout(options: TimeoutOptions): number {
+  timeout(options: { timeout?: number }): number {
     if (typeof options.timeout === 'number')
       return options.timeout;
     if (this._defaultTimeout !== null)
@@ -59,7 +58,7 @@ export class TimeoutSettings {
     return DEFAULT_TIMEOUT;
   }
 
-  static timeout(options: TimeoutOptions): number {
+  static timeout(options: { timeout?: number }): number {
     if (typeof options.timeout === 'number')
       return options.timeout;
     return DEFAULT_TIMEOUT;
diff --git a/src/utils/utils.ts b/src/utils/utils.ts
new file mode 100644
index 0000000000..b8d9e4867a
--- /dev/null
+++ b/src/utils/utils.ts
@@ -0,0 +1,126 @@
+/**
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as path from 'path';
+import * as fs from 'fs';
+import * as util from 'util';
+
+const mkdirAsync = util.promisify(fs.mkdir.bind(fs));
+
+// See https://joel.tools/microtasks/
+export function makeWaitForNextTask() {
+  if (parseInt(process.versions.node, 10) >= 11)
+    return setImmediate;
+
+  // Unlike Node 11, Node 10 and less have a bug with Task and MicroTask execution order:
+  // - https://github.com/nodejs/node/issues/22257
+  //
+  // So we can't simply run setImmediate to dispatch code in a following task.
+  // However, we can run setImmediate from-inside setImmediate to make sure we're getting
+  // in the following task.
+
+  let spinning = false;
+  const callbacks: (() => void)[] = [];
+  const loop = () => {
+    const callback = callbacks.shift();
+    if (!callback) {
+      spinning = false;
+      return;
+    }
+    setImmediate(loop);
+    // Make sure to call callback() as the last thing since it's
+    // untrusted code that might throw.
+    callback();
+  };
+
+  return (callback: () => void) => {
+    callbacks.push(callback);
+    if (!spinning) {
+      spinning = true;
+      setImmediate(loop);
+    }
+  };
+}
+
+export function assert(value: any, message?: string): asserts value {
+  if (!value)
+    throw new Error(message);
+}
+
+export function debugAssert(value: any, message?: string): asserts value {
+  if (isUnderTest() && !value)
+    throw new Error(message);
+}
+
+export function isString(obj: any): obj is string {
+  return typeof obj === 'string' || obj instanceof String;
+}
+
+export function isRegExp(obj: any): obj is RegExp {
+  return obj instanceof RegExp || Object.prototype.toString.call(obj) === '[object RegExp]';
+}
+
+export function isObject(obj: any): obj is NonNullable<object> {
+  return typeof obj === 'object' && obj !== null;
+}
+
+export function isError(obj: any): obj is Error {
+  return obj instanceof Error || (obj && obj.__proto__ && obj.__proto__.name === 'Error');
+}
+
+const isInDebugMode = !!getFromENV('PWDEBUG');
+export function isDebugMode(): boolean {
+  return isInDebugMode;
+}
+
+let _isUnderTest = false;
+export function setUnderTest() {
+  _isUnderTest = true;
+}
+export function isUnderTest(): boolean {
+  return _isUnderTest;
+}
+
+export function getFromENV(name: string) {
+  let value = process.env[name];
+  value = value || process.env[`npm_config_${name.toLowerCase()}`];
+  value = value || process.env[`npm_package_config_${name.toLowerCase()}`];
+  return value;
+}
+
+export async function mkdirIfNeeded(filePath: string) {
+  // This will harmlessly throw on windows if the dirname is the root directory.
+  await mkdirAsync(path.dirname(filePath), {recursive: true}).catch(() => {});
+}
+
+type HeadersArray = { name: string, value: string }[];
+type HeadersObject = { [key: string]: string };
+
+export function headersObjectToArray(headers: HeadersObject): HeadersArray {
+  const result: HeadersArray = [];
+  for (const name in headers) {
+    if (!Object.is(headers[name], undefined))
+      result.push({ name, value: headers[name] });
+  }
+  return result;
+}
+
+export function headersArrayToObject(headers: HeadersArray, lowerCase: boolean): HeadersObject {
+  const result: HeadersObject = {};
+  for (const { name, value } of headers)
+    result[lowerCase ? name.toLowerCase() : name] = value;
+  return result;
+}
diff --git a/src/webkit/wkBrowser.ts b/src/webkit/wkBrowser.ts
index 6b307c5ad1..962bbd42aa 100644
--- a/src/webkit/wkBrowser.ts
+++ b/src/webkit/wkBrowser.ts
@@ -17,7 +17,8 @@
 
 import { Browser, BrowserOptions } from '../browser';
 import { assertBrowserContextIsNotOwned, BrowserContext, validateBrowserContextOptions, verifyGeolocation } from '../browserContext';
-import { helper, RegisteredListener, assert } from '../helper';
+import { helper, RegisteredListener } from '../helper';
+import { assert } from '../utils/utils';
 import * as network from '../network';
 import { Page, PageBinding } from '../page';
 import * as path from 'path';
diff --git a/src/webkit/wkConnection.ts b/src/webkit/wkConnection.ts
index 15f7abae0d..5b90fa3ce8 100644
--- a/src/webkit/wkConnection.ts
+++ b/src/webkit/wkConnection.ts
@@ -16,10 +16,11 @@
  */
 
 import { EventEmitter } from 'events';
-import { assert, debugLogger } from '../helper';
+import { assert } from '../utils/utils';
 import { ConnectionTransport, ProtocolRequest, ProtocolResponse } from '../transport';
 import { Protocol } from './protocol';
 import { rewriteErrorMessage } from '../utils/stackTrace';
+import { debugLogger } from '../utils/debugLogger';
 
 // WKPlaywright uses this special id to issue Browser.close command which we
 // should ignore.
diff --git a/src/webkit/wkInput.ts b/src/webkit/wkInput.ts
index e7aa130a0f..0999131971 100644
--- a/src/webkit/wkInput.ts
+++ b/src/webkit/wkInput.ts
@@ -17,9 +17,9 @@
 
 import * as input from '../input';
 import * as types from '../types';
-import { helper } from '../helper';
 import { macEditingCommands } from '../macEditingCommands';
 import { WKSession } from './wkConnection';
+import { isString } from '../utils/utils';
 
 function toModifiersMask(modifiers: Set<types.KeyboardModifier>): number {
   // From Source/WebKit/Shared/WebEvent.h
@@ -56,7 +56,7 @@ export class RawKeyboardImpl implements input.RawKeyboard {
     parts.push(code);
     const shortcut = parts.join('+');
     let commands = macEditingCommands[shortcut];
-    if (helper.isString(commands))
+    if (isString(commands))
       commands = [commands];
     await this._pageProxySession.send('Input.dispatchKeyEvent', {
       type: 'keyDown',
diff --git a/src/webkit/wkInterceptableRequest.ts b/src/webkit/wkInterceptableRequest.ts
index 0809b66f29..1f28b278f1 100644
--- a/src/webkit/wkInterceptableRequest.ts
+++ b/src/webkit/wkInterceptableRequest.ts
@@ -16,12 +16,11 @@
  */
 
 import * as frames from '../frames';
-import { assert } from '../helper';
 import * as network from '../network';
 import * as types from '../types';
 import { Protocol } from './protocol';
 import { WKSession } from './wkConnection';
-import { headersArrayToObject, headersObjectToArray } from '../converters';
+import { assert, headersObjectToArray, headersArrayToObject } from '../utils/utils';
 
 const errorReasons: { [reason: string]: Protocol.Network.ResourceErrorType } = {
   'aborted': 'Cancellation',
diff --git a/src/webkit/wkPage.ts b/src/webkit/wkPage.ts
index 294fbff6a9..e4d1ea26d8 100644
--- a/src/webkit/wkPage.ts
+++ b/src/webkit/wkPage.ts
@@ -17,7 +17,7 @@
 
 import { Screencast, BrowserContext } from '../browserContext';
 import * as frames from '../frames';
-import { helper, RegisteredListener, assert, debugAssert } from '../helper';
+import { helper, RegisteredListener } from '../helper';
 import * as dom from '../dom';
 import * as network from '../network';
 import { WKSession } from './wkConnection';
@@ -37,7 +37,7 @@ import { selectors } from '../selectors';
 import * as jpeg from 'jpeg-js';
 import * as png from 'pngjs';
 import { JSHandle } from '../javascript';
-import { headersArrayToObject } from '../converters';
+import { assert, debugAssert, headersArrayToObject } from '../utils/utils';
 
 const UTILITY_WORLD_NAME = '__playwright_utility_world__';
 const BINDING_CALL_MESSAGE = '__playwright_binding_call__';
diff --git a/src/webkit/wkProvisionalPage.ts b/src/webkit/wkProvisionalPage.ts
index 67f5842f63..afcc5de4f2 100644
--- a/src/webkit/wkProvisionalPage.ts
+++ b/src/webkit/wkProvisionalPage.ts
@@ -16,8 +16,9 @@
 
 import { WKSession } from './wkConnection';
 import { WKPage } from './wkPage';
-import { RegisteredListener, helper, assert } from '../helper';
+import { RegisteredListener, helper } from '../helper';
 import { Protocol } from './protocol';
+import { assert } from '../utils/utils';
 
 export class WKProvisionalPage {
   readonly _session: WKSession;
diff --git a/test/fixtures/closeme.js b/test/fixtures/closeme.js
index a916c04995..76d9c82bb9 100644
--- a/test/fixtures/closeme.js
+++ b/test/fixtures/closeme.js
@@ -8,7 +8,7 @@
   }
 
   const path = require('path');
-  const { setUnderTest } = require(path.join(playwrightPath, 'lib', 'helper'));
+  const { setUnderTest } = require(path.join(playwrightPath, 'lib', 'utils', 'utils'));
   setUnderTest();
   const playwright = require(path.join(playwrightPath, 'index'));
 
diff --git a/test/playwright.fixtures.ts b/test/playwright.fixtures.ts
index 96a10b9645..5eeac93712 100644
--- a/test/playwright.fixtures.ts
+++ b/test/playwright.fixtures.ts
@@ -22,7 +22,7 @@ import { LaunchOptions, BrowserType, Browser, BrowserContext, Page, BrowserServe
 import { TestServer } from '../utils/testserver';
 import { Connection } from '../lib/rpc/client/connection';
 import { Transport } from '../lib/protocol/transport';
-import { setUnderTest } from '../lib/helper';
+import { setUnderTest } from '../lib/utils/utils';
 import { installCoverageHooks } from './coverage';
 import { parameters, registerFixture, registerWorkerFixture } from '../test-runner';
 
diff --git a/test/proxy.spec.ts b/test/proxy.spec.ts
index fb31f5e5e8..e85d999dbf 100644
--- a/test/proxy.spec.ts
+++ b/test/proxy.spec.ts
@@ -19,6 +19,13 @@ import { options } from './playwright.fixtures';
 
 import socks from 'socksv5';
 
+it('should throw for bad server value', async ({browserType, defaultBrowserOptions}) => {
+  const error = await browserType.launch({
+    ...defaultBrowserOptions,
+    proxy: { server: 123 as any }
+  }).catch(e => e);
+  expect(error.message).toContain('proxy.server: expected string, got number');
+});
 
 it('should use proxy', async ({browserType, defaultBrowserOptions, server}) => {
   server.setRoute('/target.html', async (req, res) => {
diff --git a/test/queryselector.spec.ts b/test/queryselector.spec.ts
index 511f7eaec5..588916e30b 100644
--- a/test/queryselector.spec.ts
+++ b/test/queryselector.spec.ts
@@ -17,6 +17,11 @@
 
 import './playwright.fixtures';
 
+it('should throw for non-string selector', async({page}) => {
+  const error = await page.$(null).catch(e => e);
+  expect(error.message).toContain('selector: expected string, got object');
+});
+
 it('should query existing element with css selector', async({page, server}) => {
   await page.setContent('<section>test</section>');
   const element = await page.$('css=section');
diff --git a/utils/check_deps.js b/utils/check_deps.js
index 269ffd5c5f..08fed64bf2 100644
--- a/utils/check_deps.js
+++ b/utils/check_deps.js
@@ -50,17 +50,28 @@ async function checkDeps() {
   }
 
   function allowImport(from, to) {
-    const rpc = path.join('src', 'rpc');
-    if (!from.includes(rpc) && to.includes(rpc))
+    from = from.substring(from.indexOf('src' + path.sep)).replace(/\\/g, '/');
+    to = to.substring(to.indexOf('src' + path.sep)).replace(/\\/g, '/') + '.ts';
+    while (from.lastIndexOf('/') !== -1) {
+      from = from.substring(0, from.lastIndexOf('/'));
+      const allowed = DEPS.get(from + '/');
+      if (!allowed)
+        continue;
+      for (const prefix of allowed) {
+        if (to.startsWith(prefix))
+          return true;
+      }
       return false;
-    const rpcClient = path.join('src', 'rpc', 'client');
-    const rpcServer = path.join('src', 'rpc', 'server');
-    if (from.includes(rpcClient) && to.includes(rpcServer))
-      return false;
-    // if (from.includes(rpcClient) && !to.includes(rpc))
-    //   return false;
-    return true;
+    }
+    return false;
   }
 }
 
+const DEPS = new Map([
+  ['src/utils/', ['src/utils/']],
+  ['src/protocol/', ['src/protocol/', 'src/utils/']],
+  ['src/rpc/client/', ['src/rpc/client/', 'src/utils/', 'src/protocol/', 'src/chromium/protocol.ts']],
+  ['src/', ['src/']],  // Allow everything else for now.
+]);
+
 checkDeps();