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

client/html/generated/html/frog/Window.dart

Index: client/html/generated/html/frog/Window.dart
diff --git a/client/html/generated/html/frog/Window.dart b/client/html/generated/html/frog/Window.dart
index 02edde61794f2efb905a3c961c1fa807096af605..24ce642eec4ec43ef69a79c7e24d50c59c7069e8 100644
--- a/client/html/generated/html/frog/Window.dart
+++ b/client/html/generated/html/frog/Window.dart
@@ -11,6 +11,9 @@ class _WindowImpl extends _EventTargetImpl implements Window native "@*DOMWindow
   }
 
 
+  _WindowEventsImpl get on() =>
+    new _WindowEventsImpl(this);
+
   static final int PERSISTENT = 1;
 
   static final int TEMPORARY = 0;
@@ -113,9 +116,6 @@ class _WindowImpl extends _EventTargetImpl implements Window native "@*DOMWindow
 
   final _WindowImpl window;
 
-  _WindowEventsImpl get on() =>
-    new _WindowEventsImpl(this);
-
   void _addEventListener(String type, EventListener listener, [bool useCapture = null]) native "this.addEventListener(type, listener, useCapture);";
 
   void alert(String message) native;