[Fiber] Make TreeHook DEV-only

[gaearon]

We recently started pulling TreeHook in production bundle for the sake of the Fiber stacks in error boundaries, but as @trueadm observed, this breaks the UMD bundle.

We could remove the __DEV__ gate there, but it's still bad to pull the whole hook into the prod bundle when we only use a small part of it in Fiber.

I think we should separate what we need in dev and prod, and not pull extra code into the prod bundle.

Linked: #9096

[bvaughn]

I'll look at this 😄