Revert "Switch libraries to using new tags."

sdk/lib/io/file.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;
 
 /**
  * FileMode describes the modes in which a file can be opened.
  */
 class FileMode {
   static const READ = const FileMode._internal(0);
   static const WRITE = const FileMode._internal(1);
   static const APPEND = const FileMode._internal(2);
   const FileMode._internal(int this._mode);
   final int _mode;
@@ skipping 419 matching lines @@
         sb.add(" ($osError)");
       }
     } else if (osError != null) {
       sb.add(": osError");
     }
     return sb.toString();
   }
   final String message;
   final OSError osError;
 }