Remove support for attribute event listeners.

client/dom/generated/src/frog/MessagePort.dart

 
 class MessagePort native "MessagePort" {
 
-  EventListener onmessage;
-
   void addEventListener(String type, EventListener listener, [bool useCapture = null]) native;
 
   void close() native;
 
   bool dispatchEvent(Event evt) native;
 
   void postMessage(String message, [List messagePorts = null]) native;
 
   void removeEventListener(String type, EventListener listener, [bool useCapture = null]) native;
 
   void start() native;
 
   void webkitPostMessage(String message, [List transfer = null]) native;
 
   var dartObjectLocalStorage;
 
   String get typeName() native;
 }