code: def listdir(self, target_directory):
          
          target_directory = self.resolve_path(target_directory, allow_fd=True)
          directory = self.confirmdir(target_directory)
          directory_contents = directory.contents
          return list(directory_contents.keys())
docs: Return a list of file names in target_directory.
  
          Args:
              target_directory: Path to the target directory within the
                  fake filesystem.
  
          Returns:
              A list of file names within the target directory in arbitrary
              order.
  
          Raises:
              OSError: if the target is not a directory.