Flutter Error: Constructor is marked 'const' so all fields must be final.

Listview.builder 강의에서

Stateless widget 안에 var a = 1 만들고 버튼누르면서 increment하는코드가 에러뜹니다.

lib/main.dart:8:9: Error: Constructor is marked 'const' so all fields must be final.
const MyApp({Key? key}) : super(key: key);
^
lib/main.dart:10:7: Context: Field isn't final, but constructor is 'const'.
var a = 1;

플러터버젼은 Flutter 2.10.4 입니다.

왜그러는걸까요? 

코드는 다음과같습니다.

class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);

var a = 1;

@override
Widget build(BuildContext context) {

return MaterialApp(
home: Scaffold(
floatingActionButton: FloatingActionButton(
child: Text('Click'),
onPressed: () {
a++;
print(a);
},
),
...