Revert "Switch libraries to using new tags."

sdk/lib/io/platform_impl.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;
-
 class _Platform {
   external static int _numberOfProcessors();
   external static String _pathSeparator();
   external static String _operatingSystem();
   external static _localHostname();
   external static _environment();
 
   static int get numberOfProcessors {
     return _numberOfProcessors();
   }
@@ skipping 33 matching lines @@
         if (equalsIndex == 0) {
           equalsIndex = str.indexOf('=', 1);
         }
         assert(equalsIndex != -1);
         result[str.substring(0, equalsIndex)] = str.substring(equalsIndex + 1);
       }
       return result;
     }
   }
 }