Add a sleep function to dart:io

runtime/platform/platform_headers.gypi

-# Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+# Copyright (c) 2013, 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.
 
 # This file contains headers shared between the vm and bin components.
 {
   'sources': [
     'assert.h',
     'c99_support_win.h',
     'globals.h',
     'hashmap.h',
     'inttypes_support_win.h',
     'json.h',
     'floating_point.h',
     'floating_point_win.h',
+    'platform_os.h',
     'thread.h',
     'thread_linux.h',
     'thread_macos.h',
     'thread_win.h',
     'utils.h',
   ],
 }