code: function (aNode, sourceBuffer, owner, extra) {
          var LoadingComponent = extra.ComponentClass.placeholder;
          if (typeof LoadingComponent === 'function') {
              aNodeCompiler.compileComponent(aNode, sourceBuffer, owner, {
                  ComponentClass: LoadingComponent
              });
          }
      }
docs: 编译组件加载器节点
  
  @param {ANode} aNode 节点对象
  @param {CompileSourceBuffer} sourceBuffer 编译源码的中间buffer
  @param {Component} owner 所属组件实例环境
  @param {Object} extra 编译所需的一些额外信息
  @param {Function} extra.ComponentClass 对应类