Use dart.js in release mode to workaround dartbug.com/19653

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.
 
 /// 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';
@@ skipping 60 matching lines @@
       if (options.directlyIncludeJS) {
         // If using CSP add the "precompiled" extension
         final csp = options.contentSecurityPolicy ? '.precompiled' : '';
 
         // Replace all other Dart script tags with JavaScript versions.
         for (var script in dartScripts) {
           final src = script.attributes['src'];
           if (src.endsWith('.dart')) {
             script.attributes.remove('type');
             script.attributes['src'] = '$src$csp.js';
+            // TODO(sigmund): we shouldn't need 'async' here. Remove this
+            // workaround for dartbug.com/19653.
+            script.attributes['async'] = '';
           }
         }
       } else {
         document.body.nodes.add(parseFragment(
               '<script src="packages/browser/dart.js"></script>'));
       }
 
       _addScriptFirst(urlSegment) {
         document.head.nodes.insert(0, parseFragment(
               '<script src="packages/$urlSegment"></script>\n'));
       }
 
       var suffix = options.releaseMode ? '.js' : '.concat.js';
       if (!webComponentsFound) {
         _addScriptFirst('web_components/dart_support.js');
 
         // platform.js should come before all other scripts.
         _addScriptFirst('web_components/platform$suffix');
       }
 
       transform.addOutput(
           new Asset.fromString(transform.primaryInput.id, document.outerHtml));
     });
   }
 }
 
 final _webComponentsJS = new RegExp(r'platform.*\.js',
     caseSensitive: false);