It looks like shared workers don't have a terminate method at all.

The biggest open question is how to do a runtime check against worker.
This is needed in order to adapt to the API.

It looks like that eliminates type errors. I wonder why it's not needed.

I figured out `worker` code can contain a bit of branching to take
shared workers into account. This eliminates both implementation and
test code.

Note that tests don't pass at the moment!