Fix warnings_checker.dart handling of multitests

tests/isolate/issue_21398_parent_isolate2_test.dart

 // Copyright (c) 2015, 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.
 
 // Note: the following comment is used by test.dart to additionally compile the
 // other isolate's code.
 // OtherScripts=deferred_loaded_lib.dart
 
 import 'dart:isolate';
 import 'dart:async';
@@ skipping 35 matching lines @@
           Expect.fail("We don't expect to receive any error messages");
           receivePort.close();
           asyncEnd();
         }
       );
     }
   );
 }
 
 main() {
-  helperFunction(); /// 01: runtime error
+  helperFunction(); //# 01: runtime error
 }