datastore/datastore.go
func multiKeyToProto(appID string, key []*Key) []*pb.Reference {
	ret := make([]*pb.Reference, len(key))
	for i, k := range key {
		ret[i] = keyToProto(appID, k)
	}
	return ret
}
func multiKeyToProto(appID string, key []*Key) []*pb.Reference {
	ret := make([]*pb.Reference, len(key))
	for i, k := range key {
		ret[i] = keyToProto(appID, k)
	}
	return ret
}
// multiKeyToProto is a batch version of keyToProto.