code: function nodeOwnOnlyChildrenAttach(parentEl, beforeEl) {
      this.sel = document.createComment(this.id);
      insertBefore(this.sel, parentEl, beforeEl);
  
      for (var i = 0; i < this.aNode.children.length; i++) {
          var child = createNode(
              this.aNode.children[i],
              this,
              this.childScope || this.scope,
              this.childOwner || this.owner
          );
          this.children.push(child);
          child.attach(parentEl, beforeEl);
      }
  
      this.el = document.createComment(this.id);
      insertBefore(this.el, parentEl, beforeEl);
  
      this.lifeCycle = LifeCycle.attached;
  }
docs: 将没有 root 只有 children 的元素 attach 到页面
  主要用于 slot 和 template
  
  @param {HTMLElement} parentEl 要添加到的父元素
  @param {HTMLElement＝} beforeEl 要添加到哪个元素之前