Fix directory existence query error

runtime/bin/directory_fuchsia.cc

 // Copyright (c) 2016, 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 "platform/globals.h"
 #if defined(TARGET_OS_FUCHSIA)
 
 #include "bin/directory.h"
 
 #include <dirent.h>    // NOLINT
@@ skipping 231 matching lines @@
 }
 
 
 Directory::ExistsResult Directory::Exists(const char* dir_name) {
   struct stat entry_info;
   int success = NO_RETRY_EXPECTED(stat(dir_name, &entry_info));
   if (success == 0) {
     if (S_ISDIR(entry_info.st_mode)) {
       return EXISTS;
     } else {
+      // An OSError may be constructed based on the return value of this
+      // function, so set errno to something that makes sense.
+      errno = ENOTDIR;
       return DOES_NOT_EXIST;
     }
   } else {
     if ((errno == EACCES) || (errno == EBADF) || (errno == EFAULT) ||
         (errno == ENOMEM) || (errno == EOVERFLOW)) {
       // Search permissions denied for one of the directories in the
       // path or a low level error occured. We do not know if the
       // directory exists.
       return UNKNOWN;
     }
@@ skipping 101 matching lines @@
   if (exists != EXISTS) {
     return false;
   }
   return (NO_RETRY_EXPECTED(rename(path, new_path)) == 0);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_FUCHSIA)