Fix warnings_checker.dart handling of multitests

tests/lib_strong/mirrors/library_imports_bad_metadata_test.dart

 // Copyright (c) 2014, 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.
 
 library test.library_imports_bad_metadata;
 
-@undefined // /// 01: compile-time error
+@undefined // //# 01: compile-time error
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 main() {
   LibraryMirror thisLibrary =
       currentMirrorSystem().findLibrary(#test.library_imports_bad_metadata);
 
   thisLibrary.libraryDependencies.forEach((dep) {
     Expect.listEquals([], dep.metadata);
   });
 }