code: func (ij *isolatedFile) writeJSONFile(opener openFiler) error {
  	f, err := opener.OpenFile(ij.path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
  	if err != nil {
  		return err
  	}
  	_, err = io.Copy(f, bytes.NewBuffer(ij.json))
  	err2 := f.Close()
  	if err != nil {
  		return err
  	}
  	return err2
  }
docs: // writeJSONFile writes the file contents to the filesystem.