introduce web_components pkg for consolidated polyfills

tests/html/custom_elements_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 custom_elements_test;
 import 'dart:async';
 import 'dart:html';
 import 'package:unittest/html_individual_config.dart';
 import 'package:unittest/unittest.dart';
 import 'utils.dart';
@@ skipping 23 matching lines @@
 int customCreatedCount = 0;
 
 int nextTagId = 0;
 String get nextTag => 'x-type${nextTagId++}';
 
 class NotAnElement {}
 
 main() {
   useHtmlIndividualConfiguration();
 
-  setUp(loadPolyfills);
+  setUp(() => customElementsReady);
 
   group('register', () {
     test('register', () {
       var tag = nextTag;
       document.register(tag, CustomType);
 
       var element = new Element.tag(tag);
       expect(element, isNotNull);
       expect(element is CustomType, isTrue);
       expect(element.createdCalled, isTrue);
@@ skipping 124 matching lines @@
     test('can invoke mixin methods', () {
       var tag = nextTag;
       document.register(tag, CustomType);
 
       var element = new Element.tag(tag);
       element.invokeMixinMethod();
       expect(element.mixinMethodCalled, isTrue);
     });
   });
 }