When an element changes, we should copy the source and owner again.
Otherwise they can get stale since we're not reading them from the element.

Fixes an accidental omission when both source and owner are null but displayName exists.

When we're in Fiber mode we don't actually expect that warning being printed.

We keep known keys in a set in development. There is an annoying special case where we know we'll check it again because we break out of the loop early.

One test in the tree hook regresses to the failing state because it checks that the tree hook works without a Set available, but we started using Set in this code. It is not essential and we can clean this up later when we decide how to deal with polyfills.

It needs to be available both to Fiber and Isomorphic because the tree hook lives in Isomorphic but pretty-prints Fiber stack.

The goal is to use ReactCurrentOwner less and rely on ReactDebugCurrentFiber for warning owner name and stack.

Fiber used a helper so two tests had the same phrasing.
Stack also used a helper for most invariants but hardcoded a different phrase in one place.
I changed that invariant message to use a helper which made it consistent with what it prints in Fiber.

This gets mount-time CSS warnings to be printed.

However update-time warnings are currently ignored because current fiber is not yet available during the commit phase.

We also regress on HostOperation hook tests but this doesn't matter because it's only used by ReactPerf and it doesn't work with Fiber yet anyway. We'll have to think more about it later.

This makes it available during updates, fixing the last failing test in CSSPropertyOperations.

It is not clear if the old hook system is worth it in its generic incarnation. For now I am just hooking it up to the DOMFiber renderer directly.

This helps us track which warnings are really failing in Fiber, and which ones depend on SSR.