dillon1000/react

[Fizz/Flight] Remove reentrancy hack (#22446)

* Remove reentrant check from Fizz/Flight

* Make startFlowing explicit in Flight

This is already an explicit call in Fizz. This moves flowing to be explicit.

That way we can avoid calling it in start() for web streams and therefore
avoid the reentrant call.

* Add regression test

This test doesn't actually error due to the streams polyfill not behaving
like Chrome but rather according to spec.

* Update the Web Streams polyfill

Not that we need this but just in case there are differences that are fixed.