introduce web_components pkg for consolidated polyfills

pkg/polymer/lib/polymer.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Custom HTML tags, data binding, and templates for building
  * structured, encapsulated, client-side web apps.
  *
  * Polymer.dart, the next evolution of Web UI,
  * is an in-progress Dart port of the
@@ skipping 31 matching lines @@
 import 'dart:async';
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:html';
 import 'dart:js' as js;
 
 @MirrorsUsed(metaTargets:
     const [Reflectable, ObservableProperty, CustomTag, _InitMethodAnnotation],
     override: const ['polymer', 'polymer.deserialize'])
 import 'dart:mirrors';
 
-import 'package:custom_element/polyfill.dart' show customElementsReady;
 import 'package:logging/logging.dart' show Logger, Level;
 import 'package:observe/observe.dart';
 import 'package:observe/src/dirty_check.dart' show dirtyCheckZone;
 import 'package:path/path.dart' as path;
 import 'package:polymer_expressions/polymer_expressions.dart'
     show PolymerExpressions;
 import 'package:template_binding/template_binding.dart';
+import 'package:web_components/polyfill.dart' show customElementsReady;
 
 import 'deserialize.dart' as deserialize;
 
 export 'package:observe/observe.dart';
 export 'package:observe/html.dart';
 
 part 'src/boot.dart';
 part 'src/declaration.dart';
 part 'src/instance.dart';
 part 'src/job.dart';
 part 'src/loader.dart';