introduce web_components pkg for consolidated polyfills

pkg/polymer/test/bind_mdv_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.bind_mdv_test;
 
 import 'dart:async';
 import 'dart:html';
-import 'package:custom_element/polyfill.dart';
 import 'package:template_binding/template_binding.dart';
 import 'package:observe/observe.dart';
 import 'package:unittest/html_config.dart';
 import 'package:unittest/unittest.dart';
+import 'package:web_components/polyfill.dart';
 
 main() {
   useHtmlConfiguration();
 
-  var registered = loadCustomElementPolyfill().then((_) {
+  var registered = customElementsReady.then((_) {
     document.register('my-div', MyDivElement);
   });
 
   setUp(() => registered);
 
   group('bindModel', bindModelTests);
 }
 
 bindModelTests() {
   var div;
@@ skipping 55 matching lines @@
 
 Future onAttributeChange(Element node) {
   var completer = new Completer();
   new MutationObserver((records, observer) {
     observer.disconnect();
     completer.complete();
   })..observe(node, attributes: true);
   scheduleMicrotask(Observable.dirtyCheck);
   return completer.future;
 }