Modify tests to use Directory.systemTemp

tests/standalone/io/file_test.dart

 // Copyright (c) 2013, 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 file I/O.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:collection';
 import 'dart:io';
@@ skipping 28 matching lines @@
   }
   static void asyncTestDone(String name) {
     asyncEnd();
     --numLiveAsyncTests;
     if (numLiveAsyncTests == 0) {
       deleteTempDirectory();
     }
   }
 
   static void createTempDirectory(Function doNext) {
-    new Directory('').createTemp().then((temp) {
+    Directory.systemTemp.createTemp('dart_file').then((temp) {
       tempDirectory = temp;
       doNext();
     });
   }
 
   static void deleteTempDirectory() {
     tempDirectory.deleteSync(recursive: true);
   }
 
   // Test for file read functionality.
@@ skipping 1265 matching lines @@
       testLastModified();
       testDoubleAsyncOperation();
       asyncEnd();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }