Remove disableNewFiberFeatures flag

[gaearon]

🔥 🔥 🔥 🔥 🔥

[sophiebits]

I don't understand why we had useFiber changes in the tests?

[gaearon]

These tests now fail with Stack because it has a different message. I was too lazy to fix up the message for both cases since Stack is not used.

[sophiebits]

I didn't think this commit affects Stack though. I must be missing something.

[gaearon]

The flag defaulted to false in tests.
So the tests that explicitly wanted the new code path to run had to opt in.

The message when returning undefined from component is different depending on the flag.
Some tests (that didn't opt into the new code path) asserted the old message gets thrown.

Now that there is no flag, and new code path always runs, these tests assert on the wrong message.
I changed them to assert the new message.
However, the Stack code still always emits the old message.

I could write conditionals like

expectDev(message).toContain(
  useFiber ?
    '...' :
    '...'
)

but I figured it's easier to gate the whole test (and later remove the gate).

What does this change have to do with useFiber flag though?
Well, now the only code path with old message is Stack one.
Fiber now always uses the new code path.

[sophiebits]

Ahhhhh sorry I missed that the message changed.