Fix attribute_changed_callback_test to expect event delivery either synchronously or asynchronously

tests/html/custom/attribute_changed_callback_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 attribute_changed_callback_test;
 
+import 'dart:async';
 import 'dart:html';
 import 'dart:js' as js;
 import 'package:unittest/html_individual_config.dart';
 import 'package:unittest/unittest.dart';
 import '../utils.dart';
 
 class A extends HtmlElement {
   static final tag = 'x-a';
   factory A() => new Element.tag(tag);
   A.created() : super.created();
@@ skipping 66 matching lines @@
 
       b.attributes.remove('data-v');
       expect(B.invocations, ['data-v: x => null']);
     });
 
     test('add, change ID', () {
       B.invocations = [];
 
       var b = new B();
       b.id = 'x';
-      expect(B.invocations, ['created', 'id: null => x']);
-
-      B.invocations = [];
-      b.attributes.remove('id');
-      expect(B.invocations, ['id: x => null']);
+      
+      return new Future.delayed(new Duration(milliseconds: 1))
+        .then((_) => expect(B.invocations, ['created', 'id: null => x']))
+        .then((_) {
+           B.invocations = [];
+           b.attributes.remove('id');
+         })
+        .then((_) => new Future.delayed(new Duration(milliseconds: 1)))
+        .then((_) => expect(B.invocations, ['id: x => null'])); 
     });
   });
 
   group('unsupported_on_polyfill', () {
 
     // If these tests start passing, don't remove the status suppression. Move
     // the tests to the fullYy_supported group.
 
     test('add, change classes', () {
       var b = new B();
 
       B.invocations = [];
       b.classes.toggle('u');
       expect(B.invocations, ['class: null => u']);
     });
   });
 }