https://github.com/facebook/react/blob/30b47103d4354d9187dc0f1fb804855a5208ca9f/packages/scheduler/src/Scheduler.js#L180-L185
var parentPriorityLevel = currentPriorityLevel;
  return function() {
    
    var previousPriorityLevel = currentPriorityLevel;
    currentPriorityLevel = parentPriorityLevel;
    try {
      return callback.apply(this, arguments);
    } finally {
      currentPriorityLevel = previousPriorityLevel;
    }
  };
}
function timeoutForPriorityLevel(priorityLevel) {