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