Reapply directory error handling change.

runtime/bin/platform_macos.cc

 // 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.
 
 #include "bin/platform.h"
 
+#include <string.h>
 #include <unistd.h>
 
 
 int Platform::NumberOfProcessors() {
   return sysconf(_SC_NPROCESSORS_ONLN);
 }
 
 
 const char* Platform::OperatingSystem() {
   return "macos";
 }
+
+
+char* Platform::StrError(int error_code) {
+  static const int kBufferSize = 1024;
+  char* error = static_cast<char*>(malloc(kBufferSize));
+  error[0] = '\0';
+  strerror_r(error_code, error, kBufferSize);
+  return error;
+}