Fix build breakage caused by commit 5196.

client/html/release/html.dart

Index: client/html/release/html.dart
diff --git a/client/html/release/html.dart b/client/html/release/html.dart
index 708977c61a1473cfe354e2b2e6851f65011b9384..9f7aa1f44f38686c4d46b51deff2e11577ad1459 100644
--- a/client/html/release/html.dart
+++ b/client/html/release/html.dart
@@ -6940,8 +6940,8 @@ class _DocumentEventsImpl extends _ElementEventsImpl implements DocumentEvents {
 class _DocumentFragmentImpl extends _NodeImpl implements DocumentFragment {
   _DocumentFragmentImpl._wrap(ptr) : super._wrap(ptr);
 
-  ElementEvents get on() {
-    if (_on == null) _on = new ElementEvents(this);
+  _ElementEventsImpl get on() {
+    if (_on == null) _on = new _ElementEventsImpl(this);
     return _on;
   }