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
  }
}
Assets

When a vm is present (instance creation), we need to do
a three-way merge between constructor options, instance
options and parent options.