Update dart:io tests to use asyncStart/asyncEnd

tests/standalone/io/http_client_exception_test.dart

Index: tests/standalone/io/http_client_exception_test.dart
diff --git a/tests/standalone/io/http_client_exception_test.dart b/tests/standalone/io/http_client_exception_test.dart
index 9028b6dffb1f6bca8ce8a7d4e2b649903cb385ab..e557e8d6db8e38c10c49063071264eab941c5a25 100644
--- a/tests/standalone/io/http_client_exception_test.dart
+++ b/tests/standalone/io/http_client_exception_test.dart
@@ -3,9 +3,10 @@
 // BSD-style license that can be found in the LICENSE file.
 //
 
-import "package:expect/expect.dart";
 import "dart:io";
-import "dart:isolate";
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
 
 void testInvalidUrl() {
   HttpClient client = new HttpClient();
@@ -14,13 +15,13 @@ void testInvalidUrl() {
 }
 
 void testBadHostName() {
+  asyncStart();
   HttpClient client = new HttpClient();
-  ReceivePort port = new ReceivePort();
   client.get("some.bad.host.name.7654321", 0, "/")
     .then((request) {
       Expect.fail("Should not open a request on bad hostname");
     }).catchError((error) {
-      port.close();  // We expect onError to be called, due to bad host name.
+      asyncEnd();  // We expect onError to be called, due to bad host name.
     }, test: (error) => error is! String);
 }