code: func NewDestination(dest string, fs FileSystem) TransactionalDestination {
  	if fs == nil {
  		fs = NewFileSystem(filepath.Dir(dest), "")
  	}
  	return &txnFSDest{fs: fs, dest: dest}
  }
docs: // NewDestination returns a destination in the file system (a new directory)
  // to extract a package to.
  //
  // Will use a provided FileSystem object to operate on files if given, otherwise
  // use a default one. If FileSystem is provided, dir must be in a subdirectory
  // of the given FileSystem root.