Fix warnings_checker.dart handling of multitests

tests/language_strong/mixin_illegal_object_test.dart

 // Copyright (c) 2013, 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.
 
 // Object has a non-trivial constructor and hence cannot be used as mixin.
 
 class S { }
 
 class C0 extends S
-with Object //                      /// 01: compile-time error
+with Object //                      //# 01: compile-time error
 { }
 
-class C1 = S with Object; //        /// 02: compile-time error
+class C1 = S with Object; //        //# 02: compile-time error
 
 main() {
   new C0();
-  new C1(); //                      /// 02: continued
+  new C1(); //                      //# 02: continued
 }