Adding 'part of html' to all files which are used by dart:html.

sdk/lib/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.
 
+part of html;
+
 // TODO(rnystrom): add a way to supress public classes from DartDoc output.
 // TODO(jacobr): we can remove this class now that we are using the $dom_
 // convention for deprecated methods rather than truly private methods.
 /**
  * This class is intended for testing purposes only.
  */
 class Testing {
   static void addEventListener(EventTarget target, String type, EventListener listener, bool useCapture) {
     target.$dom_addEventListener(type, listener, useCapture);
   }
   static void removeEventListener(EventTarget target, String type, EventListener listener, bool useCapture) {
     target.$dom_removeEventListener(type, listener, useCapture);
   }
 
-}
+}