code: func ReadFile(ctx context.Context, fs FileSystem, path string) ([]byte, error) {
  	rc, err := fs.Open(ctx, path)
  	if err != nil {
  		return nil, err
  	}
  	defer rc.Close()
  	return ioutil.ReadAll(rc)
  }
docs: // ReadFile reads the file named by path from fs and returns the contents.