Fixing bots after Polymer element integration

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 88 matching lines @@
     var helper = new TestHelper(phases, inputFiles)..run();
     return helper.checkAll(expectedFiles).then((_) => helper.tearDown());
   });
 }
 
 // TODO(jmesserly): this is .debug to workaround issue 13046.
 const SHADOW_DOM_TAG =
     '<script src="packages/shadow_dom/shadow_dom.debug.js"></script>\n';
 
 const INTEROP_TAG = '<script src="packages/browser/interop.js"></script>\n';
+
+const CUSTOM_ELEMENT_TAG =
+    '<script src="packages/custom_element/custom-elements.debug.js">'
+    '</script>\n';