Check that callbackAdded callbacks are only called once.

chrome/renderer/resources/extensions/apitest.js

 // Copyright (c) 2012 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.
 
 // extension_apitest.js
 // mini-framework for ExtensionApiTest browser tests
 
   chrome.test = chrome.test || {};
 
   chrome.test.tests = chrome.test.tests || [];
@@ skipping 27 matching lines @@
     // Try to get the script to stop running immediately.
     // This isn't an error, just an attempt at saying "done".
     throw "completed";
   }
 
   var pendingCallbacks = 0;
 
   chrome.test.callbackAdded = function() {
     pendingCallbacks++;
 
+    var called = false;
     return function() {
+      chrome.test.assertFalse(called, 'callback has already been run');
+      called = true;
+
       pendingCallbacks--;
       if (pendingCallbacks == 0) {
         chrome.test.succeed();
       }
     };
   };
 
   chrome.test.runNextTest = function() {
     // There may have been callbacks which were interrupted by failure
     // exceptions.
@@ skipping 202 matching lines @@
 
   chrome.test.callbackFail = function(expectedError, func) {
     return chrome.test.callback(func, expectedError);
   };
 
   chrome.test.runTests = function(tests) {
     chrome.test.tests = tests;
     testCount = chrome.test.tests.length;
     chrome.test.runNextTest();
   };