Don't put private members in dart:html interfaces.

client/html/src/_Testing.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(rnystrom): add a way to supress public classes from DartDoc output.
 /**
  * This class is intended for testing purposes only.
  */
 class Testing {
   static void addEventListener(EventTarget target, String type, EventListener listener, bool useCapture) {
-    target._addEventListener(type, listener, useCapture);
+    final _EventTargetImpl targetImpl = target;
+    targetImpl._addEventListener(type, listener, useCapture);
   }
   static void removeEventListener(EventTarget target, String type, EventListener listener, bool useCapture) {
-    target._removeEventListener(type, listener, useCapture);
+    final _EventTargetImpl targetImpl = target;
+    targetImpl._removeEventListener(type, listener, useCapture);
   }
 
 }