anchor.diff:        final subDirPath = new Path(parentDir.path).append(name);
         final subDir = new Directory.fromPath(subDirPath);
   
  -      final Map<String, dynamic> foundMap = foundContent;
  -      assert(foundMap.isEmpty);
  +      assert(foundContent == _dirPlaceHolder);
   
         return _verifyContents(subDir, expectedContent);
       } else {
  
positive.code: part of bot_io;
  
  class IoHelpers {
    static Future<bool> verifyContents(Directory dir, Map<String, dynamic> content) {
      assert(dir != null);
      assert(content != null);
  
negative.code:     lister.onDir = (String dirPath) {
        final name = new Path(dirPath).filename;
        assert(!map.containsKey(name));
        map[name] = _dirPlaceHolder;
      };
  
      lister.onDone = (bool completed) {
  