Add .fromPath constructors to File and Directory.

runtime/bin/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.
 
 
 class _Directory implements Directory {
   static final CREATE_REQUEST = 0;
   static final DELETE_REQUEST = 1;
   static final EXISTS_REQUEST = 2;
   static final CREATE_TEMP_REQUEST = 3;
   static final LIST_REQUEST = 4;
   static final RENAME_REQUEST = 5;
 
   static final SUCCESS_RESPONSE = 0;
   static final ILLEGAL_ARGUMENT_RESPONSE = 1;
   static final OSERROR_RESPONSE = 2;
 
   _Directory(String this._path);
-  _Directory.current() : _path = _current();
+  _Directory.fromPath(Path path) : this(path.toNativePath());
+  _Directory.current() : this(_current());
 
   static String _current() native "Directory_Current";
   static _createTemp(String template) native "Directory_CreateTemp";
   static int _exists(String path) native "Directory_Exists";
   static _create(String path) native "Directory_Create";
   static _delete(String path, bool recursive) native "Directory_Delete";
   static _rename(String path, String newPath) native "Directory_Rename";
   static SendPort _newServicePort() native "Directory_NewServicePort";
 
   Future<bool> exists() {
@@ skipping 254 matching lines @@
     } else {
       throw e;
     }
   }
 
   Function _onDir;
   Function _onFile;
   Function _onDone;
   Function _onError;
 }