code: func (u *FakeVolumeUtil) ReadDir(fullPath string) ([]string, error) {
  	fileNames := []string{}
  	files, found := u.directoryFiles[fullPath]
  	if !found {
  		return nil, fmt.Errorf("Directory %q not found", fullPath)
  	}
  	for _, file := range files {
  		fileNames = append(fileNames, file.Name)
  	}
  	return fileNames, nil
  }
docs: // ReadDir returns the list of all files under the given directory