code: Iterator<Path> dirIterator() {
      if (dataset.getDescriptor().isPartitioned()) {
        return Iterators.transform(partitionIterator(), new Function<StorageKey, Path>() {
          @Override
          @edu.umd.cs.findbugs.annotations.SuppressWarnings(
              value="NP_PARAMETER_MUST_BE_NONNULL_BUT_MARKED_AS_NULLABLE",
              justification="False positive, initialized above as non-null.")
          public Path apply(@Nullable StorageKey key) {
            return new Path(root, key.getPath());
          }
        });
      } else {
        return Iterators.singletonIterator(root);
      }
    }
docs: Returns an iterator that provides all leaf-level directories in this view.
  
  @return leaf-directory iterator