Revert "Switch libraries to using new tags."

sdk/lib/io/path_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 _Path implements Path {
   final String _path;
   final bool isWindowsShare;
 
   _Path(String source) : _path = source, isWindowsShare = false;
 
   _Path.fromNative(String source)
       : _path = _clean(source), isWindowsShare = _isWindowsShare(source);
 
   _Path._internal(String this._path, bool this.isWindowsShare);
@@ skipping 236 matching lines @@
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     var dirPath = (pos > 0) ? _path.substring(0, pos) : '/';
     return new _Path._internal(dirPath, isWindowsShare);
   }
 
   String get filename {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }