introduce web_components pkg for consolidated polyfills

tests/html/custom/js_custom_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 js_custom_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import '../utils.dart';
@@ skipping 10 matching lines @@
 }
 
 main() {
   useHtmlConfiguration();
 
   // Adapted from Blink's
   // fast/dom/custom/constructor-calls-created-synchronously test.
 
   var registered = false;
   setUp(() {
-    return loadPolyfills().then((_) {
+    return customElementsReady.then((_) {
       if (!registered) {
         registered = true;
         document.register(A.tag, A);
       }
     });
   });
 
   test('accessing custom Dart element from JS', () {
     var a = new A();
     a.id = 'a';
@@ skipping 21 matching lines @@
     foo.id = 'b';
     document.body.appendChild(foo);
     ''';
 
     document.body.append(new ScriptElement()..text = script);
     var custom = document.querySelector('#b');
     expect(custom is HtmlElement, isTrue);
     expect(custom.attributes['fromJS'], 'true');
   });
 }