docs/bundle.js
function flushSync(fn) {
    var previousIsBatchingUpdates = isBatchingUpdates;
    isBatchingUpdates = true;
    try {
      return syncUpdates(fn);
    } finally {
      isBatchingUpdates = previousIsBatchingUpdates;
      !!isRendering ? invariant(false, 'flushSync was called from inside a lifecycle method. It cannot be called when React is already rendering.') : void 0;
      performWork(Sync, null);
    }
  }
function flushSync(fn) {
    var previousIsBatchingUpdates = isBatchingUpdates;
    isBatchingUpdates = true;
    try {
      return syncUpdates(fn);
    } finally {
      isBatchingUpdates = previousIsBatchingUpdates;
      !!isRendering ? invariant(false, 'flushSync was called from inside a lifecycle method. It cannot be called when React is already rendering.') : void 0;
      performWork(Sync, null);
    }
  }
TODO: Batching should be implemented at the renderer level, not within the reconciler.