Allowing Window.onBeforeUnload event to work properly.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index fb6a963c1f3db8bbcd9fd94d2abc0c7b6fcc30a2..b4dfe24a01685d1ddcb5de11bd93f331616f3320 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -11113,11 +11113,11 @@ class Event extends NativeFieldWrapperClass1 {
 
   @DomName('Event.returnValue')
   @DocsEditable
-  bool get returnValue native "Event_returnValue_Getter";
+  Object get returnValue native "Event_returnValue_Getter";
 
   @DomName('Event.returnValue')
   @DocsEditable
-  void set returnValue(bool value) native "Event_returnValue_Setter";
+  void set returnValue(Object value) native "Event_returnValue_Setter";
 
   @DomName('Event.target')
   @DocsEditable
@@ -33540,7 +33540,7 @@ abstract class CssClassSet implements Set<String> {
 // BSD-style license that can be found in the LICENSE file.
 
 
-typedef void EventListener(Event event);
+typedef EventListener(Event event);
 // 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.