Summary:
changelog: [internal]
React on web uses microtasks to schedule a synchronous update for discrete event. Microtasks are not yet available in React Native and as a fallback, React uses native scheduler and task with immediate priority. Until Microtasks are in place, React Native needs to make sure all immediate tasks are executed after it dispatches each event.
I tried to stay as close to [microtask standard](https://developer.mozilla.org/en-US/docs/Web/API/queueMicrotask) as I reasonably could.
Once microtasks on RN are shipped, this code can be removed.
Reviewed By: mdvacca
Differential Revision: D33742583
fbshipit-source-id: eddebb1bd5131ee056252faad48327a70de78a4a
6351064b75