Add the ability to change the working directory

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 implements Directory {
   static const CREATE_REQUEST = 0;
   static const DELETE_REQUEST = 1;
   static const EXISTS_REQUEST = 2;
   static const CREATE_TEMP_REQUEST = 3;
   static const LIST_REQUEST = 4;
   static const RENAME_REQUEST = 5;
 
   static const SUCCESS_RESPONSE = 0;
   static const ILLEGAL_ARGUMENT_RESPONSE = 1;
   static const OSERROR_RESPONSE = 2;
 
   _Directory(String this._path);
   _Directory.fromPath(Path path) : this(path.toNativePath());
-  _Directory.current() : this(_current());
 
   external static String _current();
+  external static _setCurrent(String path);
   external static _createTemp(String template);
   external static int _exists(String path);
   external static _create(String path);
   external static _delete(String path, bool recursive);
   external static _rename(String path, String newPath);
   external static List _list(String path, bool recursive, bool followLinks);
   external static SendPort _newServicePort();
 
+  static Directory get current => new _Directory(_current());
+
+  static void set current(path) {
+    if (path is Directory) path = path.path;
+    var result = _setCurrent(path);
+    if (result is ArgumentError) throw result;
+    if (result is OSError) {
+      throw new DirectoryIOException(
+          "Setting current working directory failed", path, result);
+    }
+  }
+
   Future<bool> exists() {
     _ensureDirectoryService();
     List request = new List(2);
     request[0] = EXISTS_REQUEST;
     request[1] = _path;
     return _directoryService.call(request).then((response) {
       if (_isErrorResponse(response)) {
         throw _exceptionOrErrorFromResponse(response, "Exists failed");
       }
       return response == 1;
@@ skipping 281 matching lines @@
 
   void _ensureDirectoryService() {
     if (_directoryService == null) {
       _directoryService = _newServicePort();
     }
   }
 
   final String _path;
   SendPort _directoryService;
 }