@Override
  public synchronized Iterator<Path> iterator() {
    checkOpen();
    Iterator<Path> result = iterator;
    checkState(result != null, "iterator() has already been called once");
    iterator = null;
    return result;
  }
Secure directory stream implementation that uses a {@link FileSystemView} with the stream's
directory as its working directory.

@author Colin Decker
jimfs/src/main/java/com/google/common/jimfs/JimfsSecureDirectoryStream.java