Modify tests to use Directory.systemTemp

tests/standalone/io/regress_7679_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.
 
 import "package:expect/expect.dart";
 import 'dart:io';
 
 main() {
-  Directory temp = new Directory('').createTempSync();
+  Directory temp = Directory.systemTemp.createTempSync('dart_regress_7679');
   File script = new File('${temp.path}/script.dart');
   script.writeAsStringSync("""
 import 'dart:io';
 
 class Expect {
   static void isTrue(var x) {
     if (!identical(x, true)) {
       throw new Error("Not identical");
     }
   }
@@ skipping 14 matching lines @@
   });
 }
 """);
   String executable = new File(Platform.executable).fullPathSync();
   Process.run(executable, ['script.dart'], workingDirectory: temp.path)
       .then((result) {
         temp.deleteSync(recursive: true);
         Expect.equals(0, result.exitCode);
       });
 }