Function enqueue

docs/lib/react.js:11698–11704

* Push an update, if any, onto the queue. Creates a new queue if none is * passed and always returns the queue. Mutative.

(queue, update)

 * passed and always returns the queue. Mutative.
 */
function enqueue(queue, update) {
  if (update) {
    queue = queue || [];
    queue.push(update);
  }
  return queue;
}

/**
 * Processes any enqueued updates.