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

client/html/generated/html/dartium/XMLHttpRequest.dart

Index: client/html/generated/html/dartium/XMLHttpRequest.dart
diff --git a/client/html/generated/html/dartium/XMLHttpRequest.dart b/client/html/generated/html/dartium/XMLHttpRequest.dart
index 2ec1c1e9c4b70ef478f309a885bd387f88d4cd06..c28fa5df55cc71f85b591e458b6cbc7082b62ee0 100644
--- a/client/html/generated/html/dartium/XMLHttpRequest.dart
+++ b/client/html/generated/html/dartium/XMLHttpRequest.dart
@@ -2,6 +2,11 @@
 class _XMLHttpRequestImpl extends _EventTargetImpl implements XMLHttpRequest {
   _XMLHttpRequestImpl._wrap(ptr) : super._wrap(ptr);
 
+  _XMLHttpRequestEventsImpl get on() {
+    if (_on == null) _on = new _XMLHttpRequestEventsImpl(this);
+    return _on;
+  }
+
   bool get asBlob() => _wrap(_ptr.asBlob);
 
   void set asBlob(bool value) { _ptr.asBlob = _unwrap(value); }
@@ -30,11 +35,6 @@ class _XMLHttpRequestImpl extends _EventTargetImpl implements XMLHttpRequest {
 
   void set withCredentials(bool value) { _ptr.withCredentials = _unwrap(value); }
 
-  _XMLHttpRequestEventsImpl get on() {
-    if (_on == null) _on = new _XMLHttpRequestEventsImpl(this);
-    return _on;
-  }
-
   void abort() {
     _ptr.abort();
     return;