codeSent.dart

  codeSent() {
    return (String verificationID, [int forceResendingToken]) {
      showDialog(
          context: context,
          barrierDismissible: false,
          builder: (context) {
            return AlertDialog(
              title: Text('Enter 6-Digit Code'),
              content: Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  TextField(
                    controller: _controllerCode,
                    keyboardType: TextInputType.number,
                    decoration: InputDecoration(
                      hintText: 'Enter Code',
                    ),
                  ),

                  Container(
                    height: MediaQuery.of(context).size.height * 0.06,
                    width: MediaQuery.of(context).size.width,
                    child: FlatButton(
                      shape: RoundedRectangleBorder(
                          borderRadius: BorderRadius.circular(5)),
                      child: Text("Confirm"),
                      textColor: Colors.white,
                      color: Colors.lightBlue,
                      onPressed: () async {
                    _auth.currentUser().then((user) {
                      if (user != null) {
                        Navigator.pop(context);
                        Navigator.push(
                            context,
                            MaterialPageRoute(
                                builder: (context) => SecondScreen(
                                      user: user,
                                    )));
                      } else {
                        _verifyCode();
                      }
                    });
                      },
                    ),
                  )
                ],
              ),
            );
          });
      _verificationId = verificationID;
    };
  }