introduce web_components pkg for consolidated polyfills

samples/third_party/todomvc/test/mainpage_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 todomvc.test.mainpage_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js' as js;
 import 'package:polymer/polymer.dart';
@@ skipping 20 matching lines @@
     expect(app is TodoList, true, reason: 'TodoList should be created');
 
     final root = app.shadowRoot;
     final newTodo = root.querySelector('#new-todo');
     expect(newTodo.placeholder, "What needs to be done?");
 
     expect(app.modelId, 'model', reason: 'modelId is set via attribute');
 
     // Validate the stylesheet was loaded
     if (js.context.hasProperty('ShadowDOMPolyfill')) {
-      final style = document.head.querySelector('style[ShadowCSSShim]');
+      final style = document.head.querySelector('style[shim-shadowdom-css]');
       expect(style.text, contains('\ntd-todos #todoapp {'));
     } else {
       final style = root.querySelector('style');
       expect(style, isNotNull, reason: '<link> was replaced with <style>');
       expect(style.text, contains('\n#todoapp {'));
     }
 
     // Validate a style got applied
     var color = root.querySelector('#footer').getComputedStyle().color;
     expect(color, 'rgb(119, 119, 119)');
 
     return onPropertyInit(model, 'items').then((_) {
       expect(model.items, [], reason: 'no items yet');
       expect(app.model, model, reason: 'model should be data-bound');
     });
   });
 }