Fix warnings_checker.dart handling of multitests

tests/language_strong/prefix23_test.dart

 // Copyright (c) 2011, 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.
 //
 // Unresolved symbols should be reported as an static type warnings.
 // This should not prevent execution.
 
 library Prefix23Test.dart;
 import "library12.dart" as lib12;
 
 class myClass {
-  lib12.Library13 fld; // /// static type warning
+  lib12.Library13 fld; // //# static type warning
 }
 
 main() {
 }