introduce web_components pkg for consolidated polyfills

pkg/polymer/test/build/common.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 polymer.test.build.common;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 113 matching lines @@
 
 testPhases(String testName, List<List<Transformer>> phases,
     Map<String, String> inputFiles, Map<String, String> expectedFiles,
     [List<String> expectedMessages]) {
   test(testName, () {
     var helper = new TestHelper(phases, inputFiles, expectedMessages)..run();
     return helper.checkAll(expectedFiles).then((_) => helper.tearDown());
   });
 }
 
-// TODO(jmesserly): this is .debug to workaround issue 14720.
-const SHADOW_DOM_TAG =
-    '<script src="packages/shadow_dom/shadow_dom.debug.js"></script>\n';
+const WEB_COMPONENTS_TAG =
+    '<script src="packages/web_components/platform.js"></script>\n'
+    '<script src="packages/web_components/dart_support.js"></script>\n';
 
 const INTEROP_TAG = '<script src="packages/browser/interop.js"></script>\n';
 const DART_JS_TAG = '<script src="packages/browser/dart.js"></script>';
 
-const CUSTOM_ELEMENT_TAG =
-    '<script src="packages/custom_element/custom-elements.debug.js">'
-    '</script>\n';