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

sdk/lib/html/src/Device.dart

 // Copyright (c) 2011, 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;
+
 /**
  * Utils for device detection.
  */
 class _Device {
   /**
    * Gets the browser's user agent. Using this function allows tests to inject
    * the user agent.
    * Returns the user agent.
    */
   static String get userAgent => window.navigator.userAgent;
@@ skipping 11 matching lines @@
   /**
    * Determines if the current device is running Firefox.
    */
   static bool get isFirefox => userAgent.contains("Firefox", 0);
 
   /**
    * Determines if the current device is running WebKit.
    */
   static bool get isWebKit => !isOpera && userAgent.contains("WebKit", 0);
 }