code: function addChild(child, node) {
      var children = node.children;
      if (child.parentNode === node) {
          return;
      }
  
      children.push(child);
      child.parentNode = node;
  }
docs: It is needed to consider the mess of 'list', 'hostModel' when creating a TreeNote,
  so this function is not ready and not necessary to be public.
  
  @param {(module:echarts/data/Tree~TreeNode|Object)} child