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

client/html/generated/html/dartium/WebSocket.dart

Index: client/html/generated/html/dartium/WebSocket.dart
diff --git a/client/html/generated/html/dartium/WebSocket.dart b/client/html/generated/html/dartium/WebSocket.dart
index 7b7de0ce2ac1eca4ff266ccd36c47ed7be8b41a1..76746e397c1bb3542deb8057ba00a32369992eb5 100644
--- a/client/html/generated/html/dartium/WebSocket.dart
+++ b/client/html/generated/html/dartium/WebSocket.dart
@@ -2,6 +2,11 @@
 class _WebSocketImpl extends _EventTargetImpl implements WebSocket {
   _WebSocketImpl._wrap(ptr) : super._wrap(ptr);
 
+  _WebSocketEventsImpl get on() {
+    if (_on == null) _on = new _WebSocketEventsImpl(this);
+    return _on;
+  }
+
   String get URL() => _wrap(_ptr.URL);
 
   String get binaryType() => _wrap(_ptr.binaryType);
@@ -18,11 +23,6 @@ class _WebSocketImpl extends _EventTargetImpl implements WebSocket {
 
   String get url() => _wrap(_ptr.url);
 
-  _WebSocketEventsImpl get on() {
-    if (_on == null) _on = new _WebSocketEventsImpl(this);
-    return _on;
-  }
-
   void _addEventListener(String type, EventListener listener, [bool useCapture = null]) {
     if (useCapture === null) {
       _ptr.addEventListener(_unwrap(type), _unwrap(listener));