introduce web_components pkg for consolidated polyfills

tests/html/custom/attribute_changed_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 attribute_changed_callback_test;
 
 import 'dart:html';
 import 'dart:js' as js;
 import 'package:unittest/html_individual_config.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 31 matching lines @@
     js.context['CustomElements'].callMethod('takeRecords');
   }
 }
 
 main() {
   useHtmlIndividualConfiguration();
 
   // Adapted from Blink's fast/dom/custom/attribute-changed-callback test.
 
   var registered = false;
-  setUp(() {
-    return loadPolyfills().then((_) {
-      if (!registered) {
-        registered = true;
-        document.register(A.tag, A);
-        document.register(B.tag, B);
-      }
-    });
-  });
+  setUp(() => customElementsReady.then((_) {
+    if (!registered) {
+      registered = true;
+      document.register(A.tag, A);
+      document.register(B.tag, B);
+    }
+  }));
 
   group('fully_supported', () {
     test('transfer attribute changed callback', () {
       var element = new A();
 
       element.attributes['a'] = 'b';
       expect(A.attributeChangedInvocations, 1);
     });
 
     test('add, change and remove an attribute', () {
@@ skipping 32 matching lines @@
 
     test('add, change classes', () {
       var b = new B();
 
       B.invocations = [];
       b.classes.toggle('u');
       expect(B.invocations, ['class: null => u']);
     });
   });
 }