anchor.diff:  
     void _incrementCounter() {
       setState(() {
  -      // This call to setState tells the Flutter framework that something has
  -      // changed in this State, which causes it to rerun the build method below
  -      // so that the display can reflect the updated values. If we changed
  -      // _counter without calling setState(), then the build method would not be
  -      // called again, and so nothing would appear to happen.
         _counter++;
       });
     }
   
     @override
     Widget build(BuildContext context) {
  -    // This method is rerun every time setState is called, for instance as done
  -    // by the _incrementCounter method above.
  -    //
  -    // The Flutter framework has been optimized to make rerunning build methods
  -    // fast, so that you can just rebuild anything that needs updating rather
  -    // than having to individually change instances of widgets.
       return Scaffold(
         appBar: AppBar(
  -        // Here we take the value from the MyHomePage object that was created by
  -        // the App.build method, and use it to set our appbar title.
           title: Text(widget.title),
         ),
         body: Center(
  -        // Center is a layout widget. It takes a single child and positions it
  -        // in the middle of the parent.
           child: Column(
  -          // Column is also a layout widget. It takes a list of children and
  -          // arranges them vertically. By default, it sizes itself to fit its
  -          // children horizontally, and tries to be as tall as its parent.
  -          //
  -          // Invoke "debug painting" (press "p" in the console, choose the
  -          // "Toggle Debug Paint" action from the Flutter Inspector in Android
  -          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
  -          // to see the wireframe for each widget.
  -          //
  -          // Column has various properties to control how it sizes itself and
  -          // how it positions its children. Here we use mainAxisAlignment to
  -          // center the children vertically; the main axis here is the vertical
  -          // axis because Columns are vertical (the cross axis would be
  -          // horizontal).
             mainAxisAlignment: MainAxisAlignment.center,
             children: <Widget>[
               Text(
  
positive.code: 
  class MyHomePage extends StatefulWidget {
    MyHomePage({Key key, this.title}) : super(key: key);
  
    // This widget is the home page of your application. It is stateful, meaning
    // that it has a State object (defined below) that contains fields that affect
    // how it looks.
  
    // This class is the configuration for the state. It holds the values (in this
    // case the title) provided by the parent (in this case the App widget) and
    // used by the build method of the State. Fields in a Widget subclass are
    // always marked "final".
  
    final String title;
  
    @override
  
negative.code:     return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: MyHomePage(title: 'Flutter Demo Home Page'),
  