introduce web_components pkg for consolidated polyfills

tests/html/custom/created_callback_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 created_callback_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import 'dart:js' as js;
 import '../utils.dart';
@@ skipping 44 matching lines @@
 }
 
 main() {
   useHtmlConfiguration();
 
   // Adapted from Blink's
   // fast/dom/custom/created-callback test.
 
   var registered = false;
   setUp(() {
-    return loadPolyfills().then((_) {
+    return customElementsReady.then((_) {
       if (!registered) {
         registered = true;
         document.register(B.tag, B);
         document.register(C.tag, C);
         ErrorConstructorElement.register();
       }
     });
   });
 
   test('transfer created callback', () {
@@ skipping 214 matching lines @@
   } catch(e) {
     rethrow;
   } finally {
     js.context['testExpectsGlobalError'] = false;
   }
   var errors = js.context['testSuppressedGlobalErrors'];
   expect(errors['length'], 1);
   // Clear out the errors;
   js.context['testSuppressedGlobalErrors']['length'] = 0;
 }