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

client/html/generated/html/frog/DocumentFragment.dart

Index: client/html/generated/html/frog/DocumentFragment.dart
diff --git a/client/html/generated/html/frog/DocumentFragment.dart b/client/html/generated/html/frog/DocumentFragment.dart
index 9532ee53502740dcb43ddfeb3a4f4b2cf9b71590..dfcbf41d22e4fbe21dc47b69f9f8a6e86c1c8a72 100644
--- a/client/html/generated/html/frog/DocumentFragment.dart
+++ b/client/html/generated/html/frog/DocumentFragment.dart
@@ -1,7 +1,14 @@
 
 class _DocumentFragmentImpl extends _NodeImpl implements DocumentFragment native "*DocumentFragment" {
 
+  _DocumentFragmentEventsImpl get on() =>
+    new _DocumentFragmentEventsImpl(this);
+
   _ElementImpl query(String selectors) native "return this.querySelector(selectors);";
 
   _NodeListImpl _querySelectorAll(String selectors) native "return this.querySelectorAll(selectors);";
 }
+
+class _DocumentFragmentEventsImpl extends _EventsImpl implements DocumentFragmentEvents {
+  _DocumentFragmentEventsImpl(_ptr) : super(_ptr);
+}