Fixing BeforeUnloadEvent.returnValue

tools/dom/templates/html/impl/impl_Window.darttemplate

 // Copyright (c) 2012, 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.
 
 part of $LIBRARYNAME;
 
 @DocsEditable()
 $if DART2JS
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS native "Window,DOMWindow" {
 $else
@@ skipping 274 matching lines @@
   }
 
 $if DART2JS
   int get scrollX => JS('bool', '("scrollX" in #)', this) ? JS('int',
       '#.scrollX', this) : document.documentElement.scrollLeft;
   int get scrollY => JS('bool', '("scrollY" in #)', this) ? JS('int',
       '#.scrollY', this) : document.documentElement.scrollTop;
 $endif
 }
 
+$if DART2JS
 class _BeforeUnloadEvent extends _WrappedEvent implements BeforeUnloadEvent {
   String _returnValue;
 
   _BeforeUnloadEvent(Event base): super(base);
 
   String get returnValue => _returnValue;
 
   void set returnValue(String value) {
     _returnValue = value;
-$if DART2JS
     // FF and IE use the value as the return value, Chrome will return this from
     // the event callback function.
     if (JS('bool', '("returnValue" in #)', wrapped)) {
       JS('void', '#.returnValue = #', wrapped, value);
     }
-$endif
   }
 }
+$endif
 
 class _BeforeUnloadEventStreamProvider implements
     EventStreamProvider<BeforeUnloadEvent> {
   final String _eventType;
 
   const _BeforeUnloadEventStreamProvider(this._eventType);
 
   Stream<BeforeUnloadEvent> forTarget(EventTarget e, {bool useCapture: false}) {
+    var stream = new _EventStream(e, _eventType, useCapture);
+$if DART2JS
     var controller = new StreamController(sync: true);
-    var stream = new _EventStream(e, _eventType, useCapture);
+
     stream.listen((event) {
       var wrapped = new _BeforeUnloadEvent(event);
       controller.add(wrapped);
       return wrapped.returnValue;
     });
 
     return controller.stream;
+$else
+    return stream;
+$endif
   }
 
   String getEventType(EventTarget target) {
     return _eventType;
   }
 }