Future signIn(String email, String password) async {
FirebaseUser user = await 
FirebaseAuth.instance.signInWithEmailAndPassword(
    email: email, password: password);
return user.uid;
}

Future createUser(String email, String password) async {
FirebaseUser user = await 
FirebaseAuth.instance.createUserWithEmailAndPassword(
    email: email, password: password);
return user.uid;
}