Refactor the event-generation code to locate it in systemhtml.py.

client/html/generated/html/frog/XMLHttpRequest.dart

Index: client/html/generated/html/frog/XMLHttpRequest.dart
diff --git a/client/html/generated/html/frog/XMLHttpRequest.dart b/client/html/generated/html/frog/XMLHttpRequest.dart
index 85547f00083c204e894c2a8044066fe5463a4aa1..bd014302b812f46e66de21fa8bcf0fcdfd1e9ae4 100644
--- a/client/html/generated/html/frog/XMLHttpRequest.dart
+++ b/client/html/generated/html/frog/XMLHttpRequest.dart
@@ -1,6 +1,9 @@
 
 class _XMLHttpRequestImpl extends _EventTargetImpl implements XMLHttpRequest native "*XMLHttpRequest" {
 
+  _XMLHttpRequestEventsImpl get on() =>
+    new _XMLHttpRequestEventsImpl(this);
+
   static final int DONE = 4;
 
   static final int HEADERS_RECEIVED = 2;
@@ -35,9 +38,6 @@ class _XMLHttpRequestImpl extends _EventTargetImpl implements XMLHttpRequest nat
 
   bool withCredentials;
 
-  _XMLHttpRequestEventsImpl get on() =>
-    new _XMLHttpRequestEventsImpl(this);
-
   void abort() native;
 
   void _addEventListener(String type, EventListener listener, [bool useCapture = null]) native "this.addEventListener(type, listener, useCapture);";