Fix warnings_checker.dart handling of multitests

tests/language_strong/const_constructor_mixin2_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.
 
 class Mixin {
   var nonFinalField;
 }
 
 class A {
   const A(foo);
 }
 
 class B extends A 
-    with Mixin // /// 01: compile-time error
+    with Mixin // //# 01: compile-time error
     {
   const B(foo) : super(foo);
 }
 
 main() {
   var a = const B(42);
-  a.nonFinalField = 54; /// 01: continued
+  a.nonFinalField = 54; //# 01: continued
 }