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Currently, in a React 18 root, `act` only works if you mock the Scheduler package. This was because we didn't want to add additional checks at runtime. But now that the `act` testing API is dev-only, we can simplify its implementation. Now when an update is wrapped with `act`, React will bypass Scheduler entirely and push its tasks onto a special internal queue. Then, when the outermost `act` scope exists, we'll flush that queue. I also removed the "wrong act" warning, because the plan is to move `act` to an isomorphic entry point, simlar to `startTransition`. That's not directly related to this PR, but I didn't want to bother re-implementing that warning only to immediately remove it. I'll add the isomorphic API in a follow up. Note that the internal version of `act` that we use in our own tests still depends on mocking the Scheduler package, because it needs to work in production. I'm planning to move that implementation to a shared (internal) module, too.