Change dart:io to use Future for one-shot operations.

tests/standalone/io/many_directory_operations_test.dart

 // 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.
 //
 // Dart test program for testing using a lot of native port operations.
 
 #import("dart:io");
 
 main() {
   for (var i = 0; i < 10000; i++) {
-    Directory d = new Directory(".");
-    d.exists((result) => null);
+    new Directory(".").exists().then((result) => null);
   }
 }