Dartium fixes for latest Blink rolls.

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
 }
 
-/**
- * Event object that is fired before the window is closed.
- *
- * The standard window close behavior can be prevented by setting the
- * [returnValue]. This will display a dialog to the user confirming that they
- * want to close the page.
- */
-abstract class BeforeUnloadEvent implements Event {
-  /**
-   * If set to a non-null value, a dialog will be presented to the user
-   * confirming that they want to close the page.
-   */
-  String returnValue;
-}
-
 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
@@ skipping 21 matching lines @@
       return wrapped.returnValue;
     });
 
     return controller.stream;
   }
 
   String getEventType(EventTarget target) {
     return _eventType;
   }
 }