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

client/html/generated/html/dartium/DocumentFragment.dart

Index: client/html/generated/html/dartium/DocumentFragment.dart
diff --git a/client/html/generated/html/dartium/DocumentFragment.dart b/client/html/generated/html/dartium/DocumentFragment.dart
index f6e3dfce2a8a239a269d4602d9fdde61aa79004c..bf5b8011c997a8b50431f197dddede1387986294 100644
--- a/client/html/generated/html/dartium/DocumentFragment.dart
+++ b/client/html/generated/html/dartium/DocumentFragment.dart
@@ -2,6 +2,11 @@
 class _DocumentFragmentImpl extends _NodeImpl implements DocumentFragment {
   _DocumentFragmentImpl._wrap(ptr) : super._wrap(ptr);
 
+  _DocumentFragmentEventsImpl get on() {
+    if (_on == null) _on = new _DocumentFragmentEventsImpl(this);
+    return _on;
+  }
+
   Element query(String selectors) {
     return _wrap(_ptr.querySelector(_unwrap(selectors)));
   }
@@ -10,3 +15,7 @@ class _DocumentFragmentImpl extends _NodeImpl implements DocumentFragment {
     return _wrap(_ptr.querySelectorAll(_unwrap(selectors)));
   }
 }
+
+class _DocumentFragmentEventsImpl extends _EventsImpl implements DocumentFragmentEvents {
+  _DocumentFragmentEventsImpl(_ptr) : super(_ptr);
+}