Added async_helper and fixed broken asynchronous test.

tests/isolate/illegal_msg_test.dart

Index: tests/isolate/illegal_msg_test.dart
diff --git a/tests/isolate/illegal_msg_test.dart b/tests/isolate/illegal_msg_test.dart
index 434edc37815834842fc1127d85b7ef822d4594e6..aa17b4651b48cecabecb362b45e77d34712f9834 100644
--- a/tests/isolate/illegal_msg_test.dart
+++ b/tests/isolate/illegal_msg_test.dart
@@ -5,6 +5,7 @@
 library illegal_msg_tests;
 import "package:expect/expect.dart";
 import 'dart:isolate';
+import '../async_helper.dart';
 
 funcFoo(x) => x + 2;
 
@@ -26,8 +27,10 @@ main() {
   if (caught_exception) {
     port.close();
   } else {
+    asyncStart();
     port.receive((msg, reply) {
       print("from worker ${msg}");
+      asyncEnd();
     });
   }
   Expect.isTrue(caught_exception);