introduce web_components pkg for consolidated polyfills

pkg/polymer/lib/polymer.dart

Index: pkg/polymer/lib/polymer.dart
diff --git a/pkg/polymer/lib/polymer.dart b/pkg/polymer/lib/polymer.dart
index 8ccfaffc771e2be5e0f62e1c31967b776750972f..7aaedc07f661f1dce16fa96a2b89e103c37a47a8 100644
--- a/pkg/polymer/lib/polymer.dart
+++ b/pkg/polymer/lib/polymer.dart
@@ -49,7 +49,6 @@ import 'dart:js' as js;
     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;
@@ -57,6 +56,7 @@ 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;