introduce web_components pkg for consolidated polyfills

tests/html/custom/constructor_calls_created_synchronously_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 constructor_calls_created_synchronously_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import '../utils.dart';
 import 'dart:mirrors';
 
 class A extends HtmlElement {
   static final tag = 'x-a';
   factory A() => new Element.tag(tag);
   A.created() : super.created() {
     ncallbacks++;
   }
 
   static int ncallbacks = 0;
 }
 
 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('createdCallback', () {
     var x = new A();
     expect(A.ncallbacks, 1);
@@ skipping 16 matching lines @@
     expect(fancy.wasCreated, true, reason: 'FancySection ctor was called');
   });
 }
 
 class FancySection extends HtmlElement {
   bool wasCreated = false;
   FancySection.created() : super.created() {
     wasCreated = true;
   }
 }