title: VisualDensity is undefined?
body: <p>I've installed this package login_screen package in flutter that should unlock the visualDensity in Theme Data in flutter but It doesn't but I checked the import and package version and everything but It always give me error that It is undefined.
  Here is my code :
  pubspec.yaml file:
  environment:
    sdk: ">=2.7.0 &lt;3.0.0"</p>
  
  <pre><code>dependencies:
    flutter:
      sdk: flutter
    flutter_svg: ^0.17.4
    firebase_core: ^0.4.3+1
    firebase_auth: ^0.15.3
    modal_progress_hud: ^0.1.3
    confetti: ^0.4.0
    login_screen: ^0.6.0
  
  dev_dependencies:
    flutter_test:
      sdk: flutter
  </code></pre>
  
  <p>and my main.dart file:</p>
  
  <pre><code>import 'package:flutter/material.dart';
  import 'package:loginkit/ui/login/login.dart';
  import 'package:login_screen/login_screen.dart';
  
  void main() {
    runApp(MyApp());
  }
  
  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
          fontFamily: 'Poppins',
          visualDensity:
        ),
        debugShowCheckedModeBanner: false,
        home: Login(),
      );
    }
  }
  </code></pre>