flutter asset image not showing

flutter:  
  uses-material-design: true   
  assets:   
    - assets/

class _UserLoginState extends State<UserLogin> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: <Widget>[
          Image(image: AssetImage("assets/christmas.jpeg"),
          fit: BoxFit.cover,
        ],
      )
    );
  }
}