Improve handling of dictionary conversions.

Source/core/events/FocusEvent.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
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 [
     ConstructorTemplate=Event
 ] interface FocusEvent : UIEvent {
-    [InitializedByEventConstructor] readonly attribute EventTarget relatedTarget;
+    [InitializedByEventConstructor] readonly attribute EventTarget? relatedTarget;
 };