code: function mergeAssets (
    parentVal,
    childVal,
    vm,
    key
  ) {
    var res = Object.create(parentVal || null);
    if (childVal) {
      assertObjectType(key, childVal, vm);
      return extend(res, childVal)
    } else {
      return res
    }
  }
docs: Assets
  
  When a vm is present (instance creation), we need to do
  a three-way merge between constructor options, instance
  options and parent options.