Fix warnings_checker.dart handling of multitests

tests/isolate/simple_message_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.
 
 // Dart test program for testing that isolates are spawned.
 
 library IsolateNegativeTest;
 import "package:expect/expect.dart";
 import 'dart:isolate';
 import "package:async_helper/async_helper.dart";
 
 void entry(SendPort replyTo) {
   var message = "foo";
-  message = "bar"; // /// 01: runtime error
+  message = "bar"; // //# 01: runtime error
   replyTo.send(message);
 }
 
 main() {
   asyncStart();
   ReceivePort response = new ReceivePort();
   Isolate.spawn(entry, response.sendPort);
   response.first.then((message) {
     Expect.equals("foo", message);
     asyncEnd();
   });
 }