Improve handling of dictionary conversions.

Source/core/events/MessageEvent.cpp

Index: Source/core/events/MessageEvent.cpp
diff --git a/Source/core/events/MessageEvent.cpp b/Source/core/events/MessageEvent.cpp
index 651cec4e62fb6d0b2bc23ca05f7f553470d28148..50fe9e2229270924b14298657435a1fbbed7d3bf 100644
--- a/Source/core/events/MessageEvent.cpp
+++ b/Source/core/events/MessageEvent.cpp
@@ -28,6 +28,7 @@
 #include "config.h"
 #include "core/events/MessageEvent.h"
 
+#include "bindings/v8/ExceptionMessages.h"
 #include "core/events/ThreadLocalEventNames.h"
 #include "core/frame/DOMWindow.h"
 
@@ -133,6 +134,15 @@ MessageEvent::~MessageEvent()
 {
 }
 
+PassRefPtr<MessageEvent> MessageEvent::create(const AtomicString& type, const MessageEventInit& initializer, ExceptionState& exceptionState)
+{
+    if (initializer.source.get() && !isValidSource(initializer.source.get())) {
+        exceptionState.throwTypeError(ExceptionMessages::failedToConstruct("MessageEvent", "The optional 'source' property is neither a Window nor MessagePort."));
+        return 0;
+    }
+    return adoptRef(new MessageEvent(type, initializer));
+}
+
 void MessageEvent::initMessageEvent(const AtomicString& type, bool canBubble, bool cancelable, const String& origin, const String& lastEventId, DOMWindow* source, PassOwnPtr<MessagePortArray> ports)
 {
     if (dispatched())