Add native extensions for the Dart shell to the MacOs platform

runtime/bin/test_extension_posix.cc

 // 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.
 
 #include <string.h>
 
 #include "../include/dart_api.h"
 
 #define EXPORT_SYMBOL __attribute__ ((visibility ("default")))
 
@@ skipping 20 matching lines @@
   const char* cname;
   Dart_Handle check_error;
 
   check_error = Dart_StringToCString(name, &cname);
   if (Dart_IsError(check_error)) return NULL;
   if (!strcmp("Cat_IfNull", cname) && argc == 2) {
     return IfNull;
   }
   return NULL;
 }