Add exists, create and delete to directory implementation on Linux and Mac.

runtime/bin/directory.h

 // Copyright (c) 2011, 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.
 
 #ifndef BIN_DIRECTORY_H_
 #define BIN_DIRECTORY_H_
 
 #include "bin/builtin.h"
 #include "bin/globals.h"
 
 class Directory {
  public:
   static void List(const char* path,
                    bool recursive,
                    Dart_Port dir_port,
                    Dart_Port file_port,
                    Dart_Port done_port,
                    Dart_Port error_port);
 
+  static bool Exists(const char* path, bool* result);
+
+  static bool Create(const char* path);
+
+  static bool Delete(const char* path);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Directory);
 };
 
 #endif  // BIN_DIRECTORY_H_