Update dart:io tests to use asyncStart/asyncEnd

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";
-import 'dart:io';
-import 'dart:isolate';
 
 void main() {
   // temp/
   //   a/
   //     file.txt
   //   b/
   //     a_link -> a
   var d = new Directory("").createTempSync();
   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());
 
   // Create a symlink (or junction on Windows) from
   // temp/b/a_link to temp/a.
   var cmd = "ln";
   var args = ['-s', "${d.path}/b/a_link", "${d.path}/a"];
 
   if (Platform.operatingSystem == "windows") {
     cmd = "cmd";
     args = ["/c", "mklink", "/j", "${d.path}\\b\\a_link", "${d.path}\\a"];
   }
 
-  var keepAlive = new ReceivePort();
+  asyncStart();
 
   Process.run(cmd, args).then((_) {
     // Delete the directory containing the junction.
     b.deleteSync(recursive: true);
 
     // 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.
-    keepAlive.close();
+    asyncEnd();
   });
 }