[Extensions Bindings] Return result from event dispatch

chrome/test/data/extensions/api_test/native_bindings/extension/content_script.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-chrome.runtime.onConnect.addListener((port) => {
+chrome.runtime.onConnect.addListener(function listener(port) {
   port.onMessage.addListener((message) => {
     chrome.test.assertEq('background page', message);
     port.postMessage('content script');
   });
+  chrome.runtime.onConnect.removeListener(listener);
+});
+
+chrome.runtime.onMessage.addListener(
+    function listener(message, sender, sendResponse) {
+  chrome.test.assertEq('async bounce', message);
+  chrome.runtime.onMessage.removeListener(listener);
+  // Respond asynchronously.
+  setTimeout(() => { sendResponse('bounced'); }, 0);
+  // When returning a result asynchronously, the listener must return true -
+  // otherwise the channel is immediately closed.
+  return true;
 });
 
 chrome.runtime.sendMessage('startFlow', function(response) {
   chrome.test.assertEq('started', response);
 });