Allowing Window.onBeforeUnload event to work properly.

tools/dom/src/WrappedEvent.dart

Index: tools/dom/src/WrappedEvent.dart
diff --git a/tools/dom/src/WrappedEvent.dart b/tools/dom/src/WrappedEvent.dart
new file mode 100644
index 0000000000000000000000000000000000000000..92b13f97b97487a53dde21c2481e819b1025a0b8
--- /dev/null
+++ b/tools/dom/src/WrappedEvent.dart
@@ -0,0 +1,54 @@
+// 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.
+
+part of dart.html;
+
+/**
+ * Helper class to implement custom events which wrap DOM events.
+ */
+class _WrappedEvent implements Event {
+  final Event wrapped;
+  _WrappedEvent(this.wrapped);
+
+  bool get bubbles => wrapped.bubbles;
+
+  bool get cancelBubble => wrapped.bubbles;
+  void set cancelBubble(bool value) {
+    wrapped.cancelBubble = value;
+  }
+
+  bool get cancelable => wrapped.cancelable;
+
+  DataTransfer get clipboardData => wrapped.clipboardData;
+
+  EventTarget get currentTarget => wrapped.currentTarget;
+
+  bool get defaultPrevented => wrapped.defaultPrevented;
+
+  int get eventPhase => wrapped.eventPhase;
+
+  EventTarget get target => wrapped.target;
+
+  int get timeStamp => wrapped.timeStamp;
+
+  String get type => wrapped.type;
+
+  void $dom_initEvent(String eventTypeArg, bool canBubbleArg,
+      bool cancelableArg) {
+    throw new UnsupportedError(
+        'Cannot initialize this Event.');
+  }
+
+  void preventDefault() {
+    wrapped.preventDefault();
+  }
+
+  void stopImmediatePropagation() {
+    wrapped.stopImmediatePropagation();
+  }
+
+  void stopPropagation() {
+    wrapped.stopPropagation();
+  }
+}