.dart

import 'package:flutter/material.dart';

void main() => runApp(App());

class _AppState extends State<App> {

  int _counter = 0; 

   @override
  Widget build(BuildContext context) {

    return MaterialApp(
      title: "Counter App", 
      home: Scaffold(
          body: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
            Text("0",style: TextStyle(fontSize: 32)), 
            FlatButton(
              child: Text("Increment", style: TextStyle(color: Colors.white)), 
              onPressed: () {},
              color: Colors.green
            )
        ]),
          ),
      )
    );
    
  }

}

class App extends StatefulWidget {
  @override 
  _AppState createState() => _AppState();
}