Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/platform_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:async";
 import "dart:io";
 import "dart:isolate";
 
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
+
 test() {
   Expect.isTrue(Platform.numberOfProcessors > 0);
   var os = Platform.operatingSystem;
   Expect.isTrue(os == "android" || os == "linux" || os == "macos" ||
       os == "windows");
   Expect.equals(Platform.isLinux, Platform.operatingSystem == "linux");
   Expect.equals(Platform.isMacOS, Platform.operatingSystem == "macos");
   Expect.equals(Platform.isWindows, Platform.operatingSystem == "windows");
   Expect.equals(Platform.isAndroid, Platform.operatingSystem == "android");
   var sep = Platform.pathSeparator;
@@ skipping 33 matching lines @@
       reply.send(new Options().script);
     }
     if (msg == "close") {
       reply.send("closed");
       port.close();
     }
   });
 }
 
 testIsolate() {
-  var port = new ReceivePort();
+  asyncStart();
   var sendPort = spawnFunction(f);
   Future.wait([sendPort.call("Platform.executable"),
                sendPort.call("Platform.script"),
                sendPort.call("Platform.packageRoot"),
                sendPort.call("Platform.executableArguments"),
                sendPort.call("new Options().executable"),
                sendPort.call("new Options().script")])
   .then((results) {
     Expect.equals(Platform.executable, results[0]);
     Expect.equals(Platform.executable, results[4]);
     Uri uri = Uri.parse(results[1]);
     Expect.equals(uri, Uri.parse(results[5]));
     Expect.equals("file", uri.scheme);
     Expect.isTrue(uri.path.endsWith('tests/standalone/io/platform_test.dart'));
     Expect.equals(Platform.packageRoot, results[2]);
     Expect.listEquals(Platform.executableArguments, results[3]);
-    sendPort.call("close").then((_) => port.close());
+    sendPort.call("close").then((_) => asyncEnd());
   });
 }
 
 main() {
   test();
   testIsolate();
 }