Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/file_system_async_links_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";
-import "dart:io";
-import "dart:isolate";
-
 
 class FutureExpect {
   static Future isTrue(Future<bool> result) =>
       result.then((value) => Expect.isTrue(value));
   static Future isFalse(Future<bool> result) =>
       result.then((value) => Expect.isFalse(value));
   static Future equals(expected, Future result) =>
       result.then((value) => Expect.equals(expected, value));
   static Future listEquals(expected, Future result) =>
       result.then((value) => Expect.listEquals(expected, value));
@@ skipping 197 matching lines @@
         return true;
       }))
     .then((_) => temp.delete(recursive: true));
   });
 }
 
 
 main() {
   // Links on Windows are tested by windows_file_system_[async_]links_test.
   if (Platform.operatingSystem != 'windows') {
-    ReceivePort keepAlive = new ReceivePort();
+    asyncStart();
     testFileExistsCreate()
     .then((_) => testFileDelete())
     .then((_) => testFileWriteRead())
     .then((_) => testDirectoryExistsCreate())
     .then((_) => testDirectoryDelete())
     .then((_) => testDirectoryListing())
     .then((_) => testDirectoryListingBrokenLink())
-    .then((_) => keepAlive.close());
+    .then((_) => asyncEnd());
   }
 }