Modify tests to use Directory.systemTemp

tests/standalone/io/file_write_as_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:async';
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
@@ skipping 52 matching lines @@
           completer.complete(true);
         });
       });
     });
   });
   return completer.future;
 }
 
 main() {
   asyncStart();
-  var tempDir = new Directory('').createTempSync();
+  var tempDir = Directory.systemTemp.createTempSync('dart_file_write_as');
   testWriteAsBytesSync(tempDir);
   testWriteAsStringSync(tempDir);
   testWriteAsBytes(tempDir).then((_) {
     return testWriteAsString(tempDir);
   }).then((_) {
     tempDir.deleteSync(recursive: true);
     asyncEnd();
   });
 }