Add fromUri constructor to File, Directory and Link

sdk/lib/io/directory_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 _Directory extends FileSystemEntity implements Directory {
   final String path;
 
   _Directory(String this.path) {
     if (path is! String) {
       throw new ArgumentError('${Error.safeToString(path)} '
                               'is not a String');
     }
   }
 
+  factory _Directory.fromUri(Uri uri) => new _Directory(uri.toFilePath());
+
   external static _current();
   external static _setCurrent(path);
   external static _createTemp(String path);
   external static String _systemTemp();
   external static int _exists(String path);
   external static _create(String path);
   external static _deleteNative(String path, bool recursive);
   external static _rename(String path, String newPath);
   external static List _list(String path, bool recursive, bool followLinks);
 
@@ skipping 353 matching lines @@
       controller.addError(
           new FileSystemException("Directory listing failed",
                                  errorPath,
                                  err));
     } else {
       controller.addError(
           new FileSystemException("Internal error"));
     }
   }
 }