Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/process_pid_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.
 //
 // Process test program to test process communication.
 
-import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
-import "dart:isolate";
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
 
 main() {
-  var port = new ReceivePort();
+  asyncStart();
   Expect.isTrue(pid > 0);
   var futures = [];
   futures.add(Process.start(Platform.executable,['--version']));
   futures.add(Process.run(Platform.executable,['--version']));
   Future.wait(futures).then((results) {
     Expect.isTrue(results[0].pid > 0);
     Expect.isTrue(results[1].pid > 0);
     Expect.equals(0, results[1].exitCode);
     results[0].exitCode.then((exitCode) {
       Expect.equals(0, exitCode);
-      port.close();
+      asyncEnd();
     });
   });
 }