getFlatMenuKeys(menus) {
    let keys = [];
    menus.forEach(item => {
      if (item.children) {
        keys = keys.concat(this.getFlatMenuKeys(item.children));
      }
      keys.push(item.path);
    });
    return keys;
  }
Recursively flatten the data
[{path:string},{path:string}] => {path,path2}
@param  menus