getDocument reference

 class _ListPageState extends State<ListPage> {

  Future<DocumentReference> getPosts() async {
    final FirebaseAuth _auth = FirebaseAuth.instance;
    final Firestore _firestore = Firestore.instance;

    FirebaseUser user = await _auth.currentUser();
    DocumentReference ref = _firestore.collection('users').document(user.uid);
    return ref;
  }