Add native extensions for the Dart shell to mac and windows

runtime/bin/builtin_impl_sources.gypi

 # 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.
 
 # This file contains all C++ sources for the dart:builtin and dart:io
 # libraries.
 {
   'sources': [
     'dartutils.cc',
     'dartutils.h',
     'directory.cc',
     'directory.h',
     'directory_posix.cc',
     'directory_win.cc',
     'eventhandler.cc',
     'eventhandler.h',
     'eventhandler_linux.cc',
     'eventhandler_linux.h',
     'eventhandler_macos.cc',
     'eventhandler_macos.h',
     'eventhandler_win.cc',
     'eventhandler_win.h',
     'extensions.h',
+    'extensions.cc',
     'extensions_linux.cc',
     'extensions_macos.cc',
     'extensions_win.cc',
     'file.cc',
     'file.h',
     'file_linux.cc',
     'file_macos.cc',
     'file_win.cc',
     'file_test.cc',
     'fdutils.h',
@@ skipping 17 matching lines @@
     'socket_linux.cc',
     'socket_macos.cc',
     'socket_win.cc',
     'set.h',
     'set_test.cc',
     'thread.h',
     'thread_pool.cc',
     'thread_pool_test.cc',
   ],
 }