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

client/html/generated/html/dartium/EventSource.dart

Index: client/html/generated/html/dartium/EventSource.dart
diff --git a/client/html/generated/html/dartium/EventSource.dart b/client/html/generated/html/dartium/EventSource.dart
index 08d6618d096170233b74131435bcba473ccd2761..f6a6985c813461fecdcb3def036768a5c7862c6a 100644
--- a/client/html/generated/html/dartium/EventSource.dart
+++ b/client/html/generated/html/dartium/EventSource.dart
@@ -2,17 +2,17 @@
 class _EventSourceImpl extends _EventTargetImpl implements EventSource {
   _EventSourceImpl._wrap(ptr) : super._wrap(ptr);
 
+  _EventSourceEventsImpl get on() {
+    if (_on == null) _on = new _EventSourceEventsImpl(this);
+    return _on;
+  }
+
   String get URL() => _wrap(_ptr.URL);
 
   int get readyState() => _wrap(_ptr.readyState);
 
   String get url() => _wrap(_ptr.url);
 
-  _EventSourceEventsImpl get on() {
-    if (_on == null) _on = new _EventSourceEventsImpl(this);
-    return _on;
-  }
-
   void _addEventListener(String type, EventListener listener, [bool useCapture = null]) {
     if (useCapture === null) {
       _ptr.addEventListener(_unwrap(type), _unwrap(listener));