Modify tests to use Directory.systemTemp

tests/standalone/io/delete_symlink_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.
 
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 void main() {
   // temp/
   //   a/
   //     file.txt
   //   b/
   //     a_link -> a
-  var d = new Directory("").createTempSync();
+  var d = Directory.systemTemp.createTempSync('dart_delete_symlink');
   var a = new Directory("${d.path}/a");
   a.createSync();
 
   var b = new Directory("${d.path}/b");
   b.createSync();
 
   var f = new File("${d.path}/a/file.txt");
   f.createSync();
   Expect.isTrue(f.existsSync());
 
@@ skipping 16 matching lines @@
     // We should not have recursed through a_link into a.
     Expect.isTrue(f.existsSync());
 
     // Clean up after ourselves.
     d.deleteSync(recursive: true);
 
     // Terminate now that we are done with everything.
     asyncEnd();
   });
 }