Measure Event.returnValue usage in more detail

Source/core/events/Event.idl

 /*
  * Copyright (C) 2006, 2007, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 51 matching lines @@
     void initEvent([Default=Undefined] optional DOMString eventTypeArg,
                    [Default=Undefined] optional boolean canBubbleArg,
                    [Default=Undefined] optional boolean cancelableArg);
 
     // DOM Level 3 Additions.
     readonly attribute boolean defaultPrevented;
     void stopImmediatePropagation();
 
     // IE Extensions
     [MeasureAs=EventSrcElement] readonly attribute EventTarget srcElement;
-    [ImplementedAs=legacyReturnValue, MeasureAs=EventReturnValue] attribute boolean returnValue;
+    [CallWith=ExecutionContext, ImplementedAs=legacyReturnValue, MeasureAs=EventReturnValue] attribute boolean returnValue;
     [MeasureAs=EventCancelBubble] attribute boolean cancelBubble;
     [MeasureAs=EventPath] readonly attribute NodeList path;
     [Custom=Getter, MeasureAs=EventClipboardData] readonly attribute DataTransfer clipboardData;
 };