anchor.diff:  
   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
  
positive.code: 
    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(
  
negative.code:     return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: MyHomePage(title: 'Flutter Demo Home Page'),
  