requirejs解决异步模块加载方案

if ((!expired || usingPathFallback) && stillLoading) {
  //Something is still waiting to load. Wait for it, but only
  //if a timeout is not already in effect.
  if ((isBrowser || isWebWorker) && !checkLoadedTimeoutId) {
    checkLoadedTimeoutId = setTimeout(function () {
      checkLoadedTimeoutId = 0;
      checkLoaded();
    }, 50);
  }
}