Remove support for attribute event listeners.

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

 
 class XMLHttpRequest native "XMLHttpRequest" {
   XMLHttpRequest() native;
 
 
   bool asBlob;
 
-  EventListener onabort;
-
-  EventListener onerror;
-
-  EventListener onload;
-
-  EventListener onloadstart;
-
-  EventListener onprogress;
-
-  EventListener onreadystatechange;
-
   int readyState;
 
   Blob responseBlob;
 
   String responseText;
 
   String responseType;
 
   Document responseXML;
 
@@ skipping 22 matching lines @@
   void removeEventListener(String type, EventListener listener, [bool useCapture = null]) native;
 
   void send([var data = null]) native;
 
   void setRequestHeader(String header, String value) native;
 
   var dartObjectLocalStorage;
 
   String get typeName() native;
 }