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

client/html/generated/html/dartium/Element.dart

Index: client/html/generated/html/dartium/Element.dart
diff --git a/client/html/generated/html/dartium/Element.dart b/client/html/generated/html/dartium/Element.dart
index 18f2891ea34ecd52dff56f1a03dc9ff9219d9c2a..6df373752b5bb0e1bfb32e21a65b1af0ac5e2a20 100644
--- a/client/html/generated/html/dartium/Element.dart
+++ b/client/html/generated/html/dartium/Element.dart
@@ -539,6 +539,11 @@ class _ElementImpl extends _NodeImpl implements Element {
   }
   _ElementImpl._wrap(ptr) : super._wrap(ptr);
 
+  _ElementEventsImpl get on() {
+    if (_on == null) _on = new _ElementEventsImpl(this);
+    return _on;
+  }
+
   int get _childElementCount() => _wrap(_ptr.childElementCount);
 
   HTMLCollection get _children() => _wrap(_ptr.children);
@@ -645,11 +650,6 @@ class _ElementImpl extends _NodeImpl implements Element {
 
   void set webkitdropzone(String value) { _ptr.webkitdropzone = _unwrap(value); }
 
-  _ElementEventsImpl get on() {
-    if (_on == null) _on = new _ElementEventsImpl(this);
-    return _on;
-  }
-
   void blur() {
     _ptr.blur();
     return;