Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/file_stat_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 dart:io FileSystemEntity.Stat().
 
+import 'dart:async';
+import 'dart:io';
+
+import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 import "package:path/path.dart";
-import 'dart:async';
-import 'dart:io';
-import 'dart:isolate';
 
 void testStat() {
   Directory directory = new Directory("").createTempSync();
   File file = new File(join(directory.path, "file"));
   Expect.throws(file.statSync);
   Expect.throws(() => FileStat.statSync(file.path));
   file.writeAsStringSync("Dart IO library test of FileStat");
   new Timer(const Duration(seconds: 2), () {
     file.readAsStringSync();
     directory.listSync();
@@ skipping 90 matching lines @@
             linkStat.changed.compareTo(linkStat.accessed) < 0);
       }
       Expect.equals(7 << 6, linkStat.mode & (7 << 6));  // Includes +urwx.
       return directory.delete(recursive: true);
     });
   });
 }
 
 
 void main() {
-  ReceivePort keepAlive = new ReceivePort();
+  asyncStart();
   testStat();
-  testStatAsync().then((_) => keepAlive.close());
+  testStatAsync().then((_) => asyncEnd());
 }