Attempted fix for Firefox flakiness.

dart/tests/html/worker_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library worker_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_individual_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlIndividualConfiguration();
 
   group('supported', () {
     test('supported', () {
       expect(Worker.supported, isTrue);
     });
   });
 
   var workerScript = '''postMessage('WorkerMessage');''';
 
   group('functional', () {
 
     test('unsupported', () {
-      var expectation = Worker.supported ? returnsNormally : throws;
-
-      expect(() {
-        new Worker('worker.js');
-      }, expectation);
+      if (!Worker.supported) {
+        expect(() => new Worker('worker.js'), throws);
+      } else {
+        new Worker('worker.js').onError.first.then(expectAsync((e) {
+          // This event is expected, "worker.js" doesn't exist.  But the event
+          // *sometimes* propagates to window.onerror in Firefox which causes
+          // this test to fail, so let's stop any further propagation:
+          e.preventDefault();
+          e.stopImmediatePropagation();
+        }));
+      }
     });
 
     if (!Worker.supported) {
       return;
     }
 
     test('works', () {
       // Use Blob to make a local URL so we don't have to have a separate file.
       var blob = new Blob([workerScript], 'text/javascript');
       var url = Url.createObjectUrl(blob);
       var worker = new Worker(url);
       var test = expectAsync((e) {
         expect(e.data, 'WorkerMessage');
       });
       worker.onMessage.first.then(test);
     });
   });
 }