Allowing Window.onBeforeUnload event to work properly.

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
  Constructor]
 interface AudioContext {};
 
 [Supplemental]
 interface Document {
   [Suppressed] DOMObject getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
   CanvasRenderingContext getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
 };
 
 [Supplemental]
 interface ScriptProcessorNode {
   // FIXME(antonm): provide proper support.
   [Suppressed] attribute EventListener onaudioprocess;
 };
 
 // Force ElementTraversal. WebKit defines these directly.
 interface ElementTraversal {
   readonly attribute unsigned long childElementCount;
   readonly attribute Element firstElementChild;
   readonly attribute Element lastElementChild;
   readonly attribute Element nextElementSibling;
   readonly attribute Element previousElementSibling;
 };
 Element implements ElementTraversal;
 
+[Supplemental]
+interface Event {
+  [Suppressed] attribute boolean returnValue;
+  attribute any returnValue;
+};
+
 [Callback]
 interface TimeoutHandler {
   void handleEvent();
 };
 
 [Supplemental]
 interface Console {
   [Suppressed] void assert(in boolean condition);
   [CallWith=ScriptArguments|ScriptState] void assertCondition(boolean condition);
 };
@@ skipping 297 matching lines @@
   [Suppressed] boolean send(in DOMString data) raises(DOMException);
 
   [Custom] void send(DOMString data) raises(DOMException);
   [Custom] void send(Blob data) raises(DOMException);
   [Custom] void send(ArrayBuffer data) raises(DOMException);
   [Custom] void send(ArrayBufferView data) raises(DOMException);
 };
 
 [Suppressed]
 interface Entity {};