Fix warnings_checker.dart handling of multitests

tests/language_strong/function_type_alias9_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 for legally self referencing function type alias.
 
 typedef void F(List
-                   <G> // /// 00: compile-time error
+                   <G> // //# 00: compile-time error
                        l);
 typedef void G(List<F> l);
 
 main() {
   F foo(G g) => g;
   foo(null);
 }