Fix warnings_checker.dart handling of multitests

tests/language/prefix22_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.
 //
 // Unresolved symbols should be reported as static type warnings.
 // This should not prevent execution.
 
 library Prefix21NegativeTest.dart;
 import "library12.dart" as lib12;
 
 class myClass {
-  myClass(lib12.Library13 p) { // /// static type warning
+  myClass(lib12.Library13 p) { // //# static type warning
   }
 }
 
 main() {
   new myClass(null);  // no dynamic type error when assigning null
 }