Revert "Switch libraries to using new tags."

sdk/lib/io/platform.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 dart.io;
-
 /**
  * The [Platform] class exposes details of the machine and operating
  * system.
  */
 class Platform {
   /**
    * Get the number of processors of the machine.
    */
   static int get numberOfProcessors => _Platform.numberOfProcessors;
 
@@ skipping 12 matching lines @@
   /**
    * Get the local hostname for the system.
    */
   static String get localHostname => _Platform.localHostname;
 
   /**
    * Get the environment for this process.
    */
   static Map<String, String> get environment => _Platform.environment;
 }