fix barback and polymer build warnings

pkg/polymer/lib/src/build/polyfill_injector.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.
 
 /**
  * Final phase of the polymer transformation: includes any additional polyfills
  * that may needed by the deployed app.
  */
 library polymer.src.build.polyfill_injector;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
-import 'package:html5lib/dom.dart' show Document, Node, DocumentFragment;
+import 'package:html5lib/dom.dart' show
+    Document, DocumentFragment, Element, Node;
 import 'package:html5lib/parser.dart' show parseFragment;
 import 'common.dart';
 
 /**
  * Ensures that any scripts and polyfills needed to run a polymer application
  * are included. For example, this transformer will ensure that there is a
  * script tag that loads the shadow_dom polyfill and interop.js (used for the
  * css shimming).
  *
  * This step also replaces "packages/browser/dart.js" and the Dart script tag
@@ skipping 83 matching lines @@
 
       transform.addOutput(
           new Asset.fromString(transform.primaryInput.id, document.outerHtml));
     });
   }
 }
 
 final _shadowDomJS = new RegExp(r'shadow_dom\..*\.js', caseSensitive: false);
 final _customElementJS = new RegExp(r'custom-elements\..*\.js',
     caseSensitive: false);