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

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

 
 class _DocumentFragmentImpl extends _NodeImpl implements DocumentFragment {
   _DocumentFragmentImpl._wrap(ptr) : super._wrap(ptr);
 
+  ElementEvents get on() {
+    if (_on == null) _on = new ElementEvents(this);
+    return _on;
+  }
+
   Element query(String selectors) {
     return _wrap(_ptr.querySelector(_unwrap(selectors)));
   }
 
   NodeList _querySelectorAll(String selectors) {
     return _wrap(_ptr.querySelectorAll(_unwrap(selectors)));
   }
 }