Added async_helper and fixed broken asynchronous test.

tests/isolate/illegal_msg_stream_test.dart

Index: tests/isolate/illegal_msg_stream_test.dart
diff --git a/tests/isolate/illegal_msg_stream_test.dart b/tests/isolate/illegal_msg_stream_test.dart
index bcc91354657574b38e388c5d8fcf03eb09b9ba8f..2271ed53990670578b3a26b95205580c5a6dfdc2 100644
--- a/tests/isolate/illegal_msg_stream_test.dart
+++ b/tests/isolate/illegal_msg_stream_test.dart
@@ -6,6 +6,7 @@ library test;
 
 import 'package:expect/expect.dart';
 import 'dart:isolate';
+import '../async_helper.dart';
 
 funcFoo(x) => x + 2;
 
@@ -29,7 +30,9 @@ main() {
   snd.add(box.sink);
   snd.close();
 
+  asyncStart();
   box.stream.single.then((msg) {
     Expect.equals(499, msg);
+    asyncEnd();
   });
 }