code: function ComponentLoader(load, placeholder, fallback) {
      this.load = load;
      this.placeholder = placeholder;
      this.fallback = fallback;
  
      this.listeners = [];
  }
docs: 组件Loader类
  
  @class
  
  @param {Function} load load方法
  @param {Function=} placeholder loading过程中渲染的组件
  @param {Function=} fallback load失败时渲染的组件