Better way of fixing attribute_changed_callback_test, doesn't require a timer

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';
@@ skipping 14 matching lines @@
 
 class B extends HtmlElement {
   static final tag = 'x-b';
   factory B() => new Element.tag(tag);
   B.created() : super.created() {
     invocations.add('created');
   }
 
   static var invocations = [];
 
+  Completer completer;
+  
   void attributeChanged(name, oldValue, newValue) {
     invocations.add('$name: $oldValue => $newValue');
+    if (completer != null) {
+      completer.complete('value changed to $newValue');
+      completer = null;
+    }
   }
 }
 
 // Pump custom events polyfill events.
 void customElementsTakeRecords() {
   if (js.context.hasProperty('CustomElements')) {
     js.context['CustomElements'].callMethod('takeRecords');
   }
 }
 
@@ skipping 36 matching lines @@
       expect(B.invocations, []);
 
       b.attributes.remove('data-v');
       expect(B.invocations, ['data-v: x => null']);
     });
 
     test('add, change ID', () {
       B.invocations = [];
 
       var b = new B();
+      var completer = new Completer();
+      b.completer = completer;
       b.id = 'x';
-      
-      return new Future.delayed(new Duration(milliseconds: 1))
+      return completer.future
         .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'])); 
+          B.invocations = [];
+          var secondCompleter = new Completer();
+          b.completer = secondCompleter;
+          b.attributes.remove('id');
+          return secondCompleter.future;
+        })
+       .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']);
     });
   });
 }