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

client/html/generated/html/frog/Element.dart

Index: client/html/generated/html/frog/Element.dart
diff --git a/client/html/generated/html/frog/Element.dart b/client/html/generated/html/frog/Element.dart
index 906784ff5d547bfb34f22c897b0b8902dc7116d6..f3957924a73fe037fa093ebe5cbb69fad4651240 100644
--- a/client/html/generated/html/frog/Element.dart
+++ b/client/html/generated/html/frog/Element.dart
@@ -538,6 +538,9 @@ class _ElementImpl extends _NodeImpl implements Element native "*Element" {
         new Completer<CSSStyleDeclaration>());
   }
 
+  _ElementEventsImpl get on() =>
+    new _ElementEventsImpl(this);
+
   static final int ALLOW_KEYBOARD_INPUT = 1;
 
   int get _childElementCount() native "return this.childElementCount;";
@@ -622,9 +625,6 @@ class _ElementImpl extends _NodeImpl implements Element native "*Element" {
 
   String webkitdropzone;
 
-  _ElementEventsImpl get on() =>
-    new _ElementEventsImpl(this);
-
   void blur() native;
 
   void click() native;