code: function mergeAssets (
    parentVal,
    childVal,
    vm,
    key
  ) {
    const res = Object.create(parentVal || null)
    if (childVal) {
      process.env.NODE_ENV !== 'production' && assertObjectType(key, childVal, vm)
      return extend(res, childVal)
    }
    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.