Converting Polymer's WebComponentsLoaded to an onReady future

pkg/polymer/test/take_attributes_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.
 
-import 'dart:async';
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('x-foo')
 class XFoo extends PolymerElement {
   XFoo.created() : super.created();
 
   @published bool boolean = false;
@@ skipping 30 matching lines @@
 
 @CustomTag('x-obj')
 class XObj extends PolymerElement {
   XObj.created() : super.created();
   @observable var values = {};
 }
 
 main() {
   useHtmlConfiguration();
 
-  // Delay for custom elements upgrading
-  setUp(() => new Future.delayed(Duration.ZERO));
+  setUp(() => Polymer.onReady);
 
   test('take attributes', () {
     queryXTag(x) => document.query(x).xtag;
 
     expect(queryXTag("#foo0").boolean, true);
     expect(queryXTag("#foo1").boolean, false);
     expect(queryXTag("#foo2").boolean, true);
     expect(queryXTag("#foo3").boolean, false);
     // this one is only 'truthy'
     expect(queryXTag("#foo4").boolean, true);
@@ skipping 46 matching lines @@
     expect(queryXTag("#arr1").values, [0, 1, 2]);
     expect(queryXTag("#arr2").values, [33]);
     // Object deserialization tests
     expect(queryXTag("#obj1").values, { 'name': 'Brandon',
         'nums': [1, 22, 33] });
     expect(queryXTag("#obj2").values, { "color": "Red" });
     expect(queryXTag("#obj3").values, { 'movie': 'Buckaroo Banzai',
         'DOB': '07/31/1978' });
   });
 }