환경

environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter


  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.3
  firebase_core: ^0.7.0
  cloud_firestore: ^0.16.0+1
  firebase_auth: ^0.20.1

The argument type 'Stream' can't be assigned to the parameter type 'Future'.

 Widget build(BuildContext context) {
    return FutureBuilder(
        future: FirebaseAuth.instance.currentUser, // ここでエラー
        builder: (ctx, futureSnapshot) {
          if (futureSnapshot.hasData) {
            User user = futureSnapshot.data;
            return Dashboard();
          }
          return LoginScreen();
        });
  }

StreamBuilder와 authStateChanges로 대체?

 Widget build(BuildContext context) {
    return StreamBuilder(
        stream: FirebaseAuth.instance.authStateChanges(),
        builder: (ctx, futureSnapshot) {
          if (futureSnapshot.hasData) {
            User user = futureSnapshot.data;
            return Dashboard();
          }
          return LoginScreen();
        });
  }