True. Cite: https://wiki.mozilla.org/WebAPI

They plan FileReader and FileWriter though.

Now I wonder: a) What is Moz leaving out? Why? b) Which of the other items from FileSystem does the Google team need, if any?

It looks like they're leaving out everything in the spec I linked above. Currently this is the only way to persist File objects locally, although it looks like they're planning to make that possible through IndexedDB instead.

Do you have any source?