How to remove default navigation route animation

Replace your MyApp with this.

class MyApp extends StatelessWidget {
  final routes = <String, WidgetBuilder>{SecondRoute.tag: (context) => SecondRoute()};

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "Flutter Routes",
      home: new FirstRoute(),
      onGenerateRoute: (routeSettings) {
        if (routeSettings.name == SecondRoute.tag)
          return PageRouteBuilder(
            pageBuilder: (_, a1, a2) => FadeTransition(opacity: a1 ,child: SecondRoute()),
            transitionDuration: Duration(seconds: 5),
          );

        return null;
      },
    );
  }
}

• For Navigator.push(...)

  Navigator.push(
    context,
    PageRouteBuilder(pageBuilder: (_, __, ___) => SecondRoute()),
  )
  

• For Navigator.pushNamed(...)

  First, add this to your MaterialApp

  MaterialApp(
    onGenerateRoute: (settings) {
      if (settings.name == '/second') 
        return PageRouteBuilder(pageBuilder: (_, __, ___) => SecondRoute());
  
      return null;
    },
  )
  

  And now, you can use:

  Navigator.pushNamed(context, '/second');
  

The animation is performed by MaterialPageRoute. If you don't want it, simple use something else:

Navigator.push(
  context,
  PageRouteBuilder(pageBuilder: (_, __, ___) => MyRoute()),
)