Change Directory.create to not fail on existing directory with that name.

runtime/bin/directory_macos.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 "bin/directory.h"
 
 #include <dirent.h>
 #include <errno.h>
 #include <sys/param.h>
 #include <sys/stat.h>
@@ skipping 363 matching lines @@
   }
 }
 
 
 char* Directory::Current() {
   return getcwd(NULL, 0);
 }
 
 
 bool Directory::Create(const char* dir_name) {
+  // If the directory already exists and is a directory do not
+  // attempt to create it again and treat it as a success.
+  if (Exists(dir_name) == EXISTS) return true;
   // Create the directory with the permissions specified by the
   // process umask.
   return (TEMP_FAILURE_RETRY(mkdir(dir_name, 0777)) == 0);
 }
 
 
 char* Directory::CreateTemp(const char* const_template) {
   // Returns a new, unused directory name, modifying the contents of
   // dir_template.  Creates the directory with the permissions specified
   // by the process umask.
@@ skipping 29 matching lines @@
     return DeleteRecursively(dir_name);
   }
 }
 
 
 bool Directory::Rename(const char* path, const char* new_path) {
   ExistsResult exists = Exists(path);
   if (exists != EXISTS) return false;
   return (TEMP_FAILURE_RETRY(rename(path, new_path)) == 0);
 }