introduce web_components pkg for consolidated polyfills

tests/html/custom/document_register_basic_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 document_register_basic_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import '../utils.dart';
 
@@ skipping 27 matching lines @@
 class BadE implements HtmlElement {
   static final tag = 'x-tag-e';
   factory BadE() => new Element.tag(tag);
 }
 
 main() {
   useHtmlConfiguration();
 
   // Adapted from Blink's fast/dom/custom/document-register-basic test.
 
-  setUp(loadPolyfills);
+  setUp(() => customElementsReady);
 
   test('Testing document.register() basic behaviors', () {
     document.register(Foo.tag, Foo);
 
     // Cannot register an existing dart:html type.
     expect(() => document.register('x-bad-a', HtmlElement), throws);
 
     // Invalid user type.  Doesn't inherit from HtmlElement.
     expect(() => document.register('x-bad-b', BadB), throws);
 
@@ skipping 73 matching lines @@
     expect(container.firstChild.tagName, "X-BAR");
     expect(container.lastChild is Bar, isTrue);
     expect(container.lastChild.tagName, "X-BAR");
 
     // Constructors shouldn't interfere with each other
     expect((new Foo()).tagName, "X-FOO");
     expect((new Bar()).tagName, "X-BAR");
     expect((new Baz()).tagName, "X-BAZ");
   });
 }