Fix warnings_checker.dart handling of multitests

tests/language/const_native_factory_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.
 
 class Cake {
   final name;
   const Cake(this.name);
-  const factory BakeMeACake() native "Cake_BakeMeACake"; // /// 01: compile-time error
+  const factory BakeMeACake() native "Cake_BakeMeACake"; // //# 01: compile-time error
 }
 
 main() {
   var c = const Cake("Sacher");
 }