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

runtime/bin/directory_win.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 <errno.h>
 #include <sys/stat.h>
 
 #include "bin/platform.h"
@@ skipping 330 matching lines @@
 char* Directory::Current() {
   char* result;
   int length = GetCurrentDirectory(0, NULL);
   result = reinterpret_cast<char*>(malloc(length + 1));
   GetCurrentDirectory(length + 1, result);
   return result;
 }
 
 
 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;
   return (CreateDirectory(dir_name, NULL) != 0);
 }
 
 
 char* Directory::CreateTemp(const char* const_template) {
   // Returns a new, unused directory name, modifying the contents of
   // dir_template.  Creates this directory, with a default security
   // descriptor inherited from its parent directory.
   // The return value must be freed by the caller.
   char* path = static_cast<char*>(malloc(MAX_PATH));
@@ skipping 57 matching lines @@
   // MoveFile does not allow replacing exising directories. Therefore,
   // if the new_path is currently a directory we need to delete it
   // first.
   if (new_exists == EXISTS) {
     bool success = DeleteRecursively(new_path);
     if (!success) return false;
   }
   DWORD flags = MOVEFILE_WRITE_THROUGH;
   return (MoveFileEx(path, new_path, flags) != 0);
 }