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

client/html/generated/html/dartium/Window.dart

Index: client/html/generated/html/dartium/Window.dart
diff --git a/client/html/generated/html/dartium/Window.dart b/client/html/generated/html/dartium/Window.dart
index 10f90b68afeaa35b43f1ea774849f4d6aa5f6730..f66d28c4ccbb7e0bd6d24899651fe3b632b88ae7 100644
--- a/client/html/generated/html/dartium/Window.dart
+++ b/client/html/generated/html/dartium/Window.dart
@@ -12,6 +12,11 @@ class _WindowImpl extends _EventTargetImpl implements Window {
 
   _WindowImpl._wrap(ptr) : super._wrap(ptr);
 
+  _WindowEventsImpl get on() {
+    if (_on == null) _on = new _WindowEventsImpl(this);
+    return _on;
+  }
+
   DOMApplicationCache get applicationCache() => _wrap(_ptr.applicationCache);
 
   Navigator get clientInformation() => _wrap(_ptr.clientInformation);
@@ -120,11 +125,6 @@ class _WindowImpl extends _EventTargetImpl implements Window {
 
   Window get window() => _wrap(_ptr.window);
 
-  _WindowEventsImpl get on() {
-    if (_on == null) _on = new _WindowEventsImpl(this);
-    return _on;
-  }
-
   void _addEventListener(String type, EventListener listener, [bool useCapture = null]) {
     if (useCapture === null) {
       _ptr.addEventListener(_unwrap(type), _unwrap(listener));