anchor.diff:          if (child is File) {
             var path = child.path;
             if (file_paths.isDart(pathContext, path)) {
  -            files!.add(path);
  +            files.add(path);
             }
           } else if (child is Folder) {
             _appendFilesRecursively(child);
  
positive.code:     }
      _discoverAvailableFiles();
      _scheduler.notify(this);
      return _discoverAvailableFilesTask!.completer!.future;
    }
  
    @override
  
negative.code:     }
  
      // Get know files one by one.
      while (fileIndex < files.length) {
        if (timer.elapsedMilliseconds > _MS_WORK_INTERVAL) {
          return;
        }
        var file = files[fileIndex++];
        driver._fsState.getFileForPath(file);
      }
  
      // The task is done, clean up.
      folderIterator = null;
      files.clear();
      completer.complete();
    }
  
    void _appendFilesRecursively(Folder folder) {
  