Allowing Window.onBeforeUnload event to work properly.

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;
 
 $if DART2JS
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS native "@*DOMWindow" {
 $else
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
@@ skipping 213 matching lines @@
     throw new UnsupportedError('setImmediate is not supported');
   }
 $endif
 
   /**
    * Access a sandboxed file system of the specified `size`. If `persistent` is
    * true, the application will request permission from the user to create
    * lasting storage. This storage cannot be freed without the user's
    * permission. Returns a [Future] whose value stores a reference to the
    * sandboxed file system for use. Because the file system is sandboxed,
-   * applications cannot access file systems created in other web pages. 
+   * applications cannot access file systems created in other web pages.
    */
   Future<FileSystem> requestFileSystem(int size, {bool persistent: false}) {
     return _requestFileSystem(persistent? 1 : 0, size);
   }
 $!MEMBERS
+
+  @DomName('DOMWindow.beforeunloadEvent')
+  @DocsEditable
+  static const EventStreamProvider<BeforeUnloadEvent> beforeUnloadEvent =
+      const _BeforeUnloadEventStreamProvider('beforeunload');
+
+  @DomName('DOMWindow.onbeforeunload')
+  @DocsEditable
+  Stream<Event> get onBeforeUnload => beforeUnloadEvent.forTarget(this);
 }
+
+/**
+ * Event object that is fired before the window is closed.
+ *
+ * The standard window close behavior can be prevented by setting the
+ * [returnValue]. This will display a dialog to the user confirming that they
+ * want to close the page.
+ */
+abstract class BeforeUnloadEvent implements Event {
+  /**
+   * If set to a non-null value, a dialog will be presented to the user
+   * confirming that they want to close the page.
+   */
+  String returnValue;
+}
+
+class _BeforeUnloadEvent extends _WrappedEvent implements BeforeUnloadEvent {
+  String _returnValue;
+
+  _BeforeUnloadEvent(Event base): super(base);
+
+  String get returnValue => _returnValue;
+
+  void set returnValue(String value) {
+    _returnValue = value;
+$if DART2JS
+    // FF and IE use the value as the return value, Chrome will return this from
+    // the event callback function.
+    if (JS('bool', '("returnValue" in #)', _base)) {
+      JS('void', '#.returnValue = #', _base, value);
+    }
+$endif
+  }
+}
+
+class _BeforeUnloadEventStreamProvider implements
+    EventStreamProvider<BeforeUnloadEvent> {
+  final String _eventType;
+
+  const _BeforeUnloadEventStreamProvider(this._eventType);
+
+  Stream<BeforeUnloadEvent> forTarget(EventTarget e, {bool useCapture: false}) {
+    var controller = new StreamController.broadcast();
+    var stream = new _EventStream(e, _eventType, useCapture);
+    stream.listen((event) {
+      var wrapped = new _BeforeUnloadEvent(event);
+      controller.add(wrapped);
+      return wrapped.returnValue;
+    });
+
+    return controller.stream;
+  }
+
+  String getEventType(EventTarget target) {
+    return _eventType;
+  }
+}