Everything is specialized for BodyStream anyway, so this base class is superfluous.

I thought the idea was that NativeUnderlyingSource will be used by other users in the near future.

In theory, yes. In practice, it's insufficiently well proven to support that use right now, and I would rather someone trying to make a new consumer here not be confused into thinking that this is just going to work because it really might not; step one for trying to add a new consumer may well be to revert this removal, but I'm very supportive of removing this until the story is clearer.

Jan, you may want to take a look at this change. I assume we'll need something like NativeUnderlyingSource for OPFS.

Hard to say at this point, we are currently implementing only basic functionality of OPFS. I will know more once we get to implementing stream support.
Anyway, thanks for letting me know.

https://hg.mozilla.org/mozilla-central/rev/0d726177468a