[polymer.dart] fix *Changed observers inheriting

pkg/polymer/test/event_handlers_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 polymer.test.event_handlers_test;
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:logging/logging.dart';
@@ skipping 98 matching lines @@
   initPolymer();
 }
 
 @initMethod _init() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
   test('events handled', () {
     XTest test = query('x-test');
     expect(test._onTestDone, isNotNull, reason: 'ready was called');
+    return test._onTestDone;
   });
 }