Use userAgent instead of webkitStartDart

pkg/browser/lib/interop.js

 // 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.
 
 // ---------------------------------------------------------------------------
 // Support for JS interoperability
 // ---------------------------------------------------------------------------
 function SendPortSync() {
 }
 
@@ skipping 146 matching lines @@
   };
 
   ReceivePortSync.prototype.toSendPort = function() {
     return new LocalSendPortSync(this);
   };
 
   ReceivePortSync.prototype.close = function() {
     delete ReceivePortSync.map[this.id];
   };
 
-  if (navigator.webkitStartDart) {
+  if (navigator.userAgent.indexOf('(Dart)') !== -1) {
     window.addEventListener('js-sync-message', function(event) {
       var data = JSON.parse(getPortSyncEventData(event));
       var deserialized = deserialize(data.message);
       var result = ReceivePortSync.map[data.id].callback(deserialized);
       // TODO(vsm): Handle and propagate exceptions.
       dispatchEvent('js-result', serialize(result));
     }, false);
   }
 
   function LocalSendPortSync(receivePort) {
@@ skipping 350 matching lines @@
     port.receive(f);
     window.registerPort(name, port.toSendPort());
   }
 
   makeGlobalPort('dart-js-context', context);
   makeGlobalPort('dart-js-create', construct);
   makeGlobalPort('dart-js-instanceof', proxyInstanceof);
   makeGlobalPort('dart-js-delete-property', proxyDeleteProperty);
   makeGlobalPort('dart-js-convert', proxyConvert);
 })();