Add the ability to get the current 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.
 
 
 // Used for holding error code and error message for failed OS system calls.
 class _OSStatus {
   int _errorCode;
   String _errorMessage;
 }
 
 
 class _Directory implements Directory {
   static final kCreateRequest = 0;
   static final kDeleteRequest = 1;
   static final kExistsRequest = 2;
   static final kCreateTempRequest = 3;
   static final kListRequest = 4;
 
   _Directory(String this._path);
+  _Directory.current() : _path = _current();
 
+  static String _current() native "Directory_Current";
   static String _createTemp(String template,
                             _OSStatus status) native "Directory_CreateTemp";
   static int _exists(String path) native "Directory_Exists";
   static bool _create(String path) native "Directory_Create";
   static bool _delete(String path, bool recursive) native "Directory_Delete";
   static SendPort _newServicePort() native "Directory_NewServicePort";
 
   void exists(void callback(bool exists)) {
     _ensureDirectoryService();
     List request = new List(2);
@@ skipping 168 matching lines @@
   }
 
   var _onDir;
   var _onFile;
   var _onDone;
   var _onError;
 
   String _path;
   SendPort _directoryService;
 }