DocumentReference.parent() unexpectedly returning null

[suztomo]

This test fails at 0.4.2:

  test('Document reference path', () async {
    final instance = MockFirestoreInstance();
    final documentReference = instance
        .collection('users')
        .document('aaa')
        .collection('friends')
        .document('bbb')
        .collection('friends-friends')
        .document('ccc');

    final friendsFriends = documentReference.parent(); // null
    final bbb = friendsFriends.parent();
    final friends = bbb.parent();
    final bbbSibling = friends.document('bbb-sibling');
    expect(bbbSibling.path, 'users/aaa/friends/bbb-sibling');
  });