dillon1000/react

Make root.unmount() synchronous (#22444)

* Move flushSync warning to React DOM

When you call in `flushSync` from an effect, React fires a warning. I've
moved the implementation of this warning out of the reconciler and into
React DOM.

`flushSync` is a renderer API, not an isomorphic API, because it has
behavior that was designed specifically for the constraints of React
DOM. The equivalent API in a different renderer may not be the same.
For example, React Native has a different threading model than the
browser, so it might not make sense to expose a `flushSync` API to the
JavaScript thread.

* Make root.unmount() synchronous

When you unmount a root, the internal state that React stores on the
DOM node is immediately cleared. So, we should also synchronously
delete the React tree. You should be able to create a new root using
the same container.