Improve handling of dictionary conversions.

Source/core/events/MessageEvent.idl

Index: Source/core/events/MessageEvent.idl
diff --git a/Source/core/events/MessageEvent.idl b/Source/core/events/MessageEvent.idl
index 417e2b6ef6a2c5b0550d9837691b684fa67e77e0..5bdda40a2f14f87df49a7a9b9e83cc4cb9e62902 100644
--- a/Source/core/events/MessageEvent.idl
+++ b/Source/core/events/MessageEvent.idl
@@ -27,14 +27,14 @@
 
 [
     GlobalContext=Window&WorkerGlobalScope,
-    ConstructorTemplate=Event
+    ConstructorTemplate=Event,
+    RaisesException=Constructor
 ] interface MessageEvent : Event {
     [InitializedByEventConstructor] readonly attribute DOMString origin;
     [InitializedByEventConstructor] readonly attribute DOMString lastEventId;
-    [InitializedByEventConstructor] readonly attribute EventTarget source; // May be a Window or a MessagePort
+    [InitializedByEventConstructor] readonly attribute EventTarget? source; // May be a Window or a MessagePort
     [InitializedByEventConstructor, Custom=Getter] readonly attribute any data;
     [InitializedByEventConstructor] readonly attribute MessagePort[] ports;
-
     [Custom] void initMessageEvent([Default=Undefined] optional DOMString typeArg,
                                    [Default=Undefined] optional boolean canBubbleArg,
                                    [Default=Undefined] optional boolean cancelableArg,