code: function AsyncComponent(options, loader) {
      this.options = options;
      this.loader = loader;
      this.id = guid++;
      this.children = [];
  
      // #[begin] reverse
      var reverseWalker = options.reverseWalker;
      if (reverseWalker) {
          var PlaceholderComponent = this.loader.placeholder;
          if (PlaceholderComponent) {
              this.children[0] = new PlaceholderComponent(options);
          }
  
          this._create();
          insertBefore(this.el, reverseWalker.target, reverseWalker.current);
  
          var me = this;
          this.loader.start(function (ComponentClass) {
              me.onload(ComponentClass);
          });
      }
      options.reverseWalker = null;
      // #[end]
  }
docs: 异步组件类
  
  @class
  @param {Object} options 初始化参数
  @param {Object} loader 组件加载器