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

client/html/generated/html/frog/EventSource.dart

Index: client/html/generated/html/frog/EventSource.dart
diff --git a/client/html/generated/html/frog/EventSource.dart b/client/html/generated/html/frog/EventSource.dart
index d4cb4eb17b18145ff13c0efe3ac75603e76b1fbb..315880844c62711e9b1507e0eac300b3f98b56d6 100644
--- a/client/html/generated/html/frog/EventSource.dart
+++ b/client/html/generated/html/frog/EventSource.dart
@@ -1,6 +1,9 @@
 
 class _EventSourceImpl extends _EventTargetImpl implements EventSource native "*EventSource" {
 
+  _EventSourceEventsImpl get on() =>
+    new _EventSourceEventsImpl(this);
+
   static final int CLOSED = 2;
 
   static final int CONNECTING = 0;
@@ -13,9 +16,6 @@ class _EventSourceImpl extends _EventTargetImpl implements EventSource native "*
 
   final String url;
 
-  _EventSourceEventsImpl get on() =>
-    new _EventSourceEventsImpl(this);
-
   void _addEventListener(String type, EventListener listener, [bool useCapture = null]) native "this.addEventListener(type, listener, useCapture);";
 
   void close() native;