Add Event.returnValue.

dom.bs

@@ -517,6 +517,7 @@ interface Event {
 
   readonly attribute boolean bubbles;
   readonly attribute boolean cancelable;
+           attribute boolean returnValue;
   void preventDefault();
   readonly attribute boolean defaultPrevented;
   readonly attribute boolean composed;
@@ -735,6 +736,12 @@ The <dfn attribute for=Event><code>bubbles</code></dfn> and
 <dfn attribute for=Event><code>cancelable</code></dfn> attributes
 must return the values they were initialized to.
 
+The <dfn attribute for=Event><code>returnValue</code></dfn>
+attribute must be initialized to true when an <a>event</a> is created.
+When the attribute is set to false it must set the <a>canceled flag</a> if the
+{{Event/cancelable}} attribute value is true and the
+<a>in passive listener flag</a> is unset.
+
 The <dfn method for=Event><code>preventDefault()</code></dfn> method, when invoked, must set the
 <a>canceled flag</a> if the {{Event/cancelable}} attribute value is true and the
 <a>in passive listener flag</a> is unset.