add navigator to dart file that has no widgets in flutter code example

Example 1: flutter push route

// Within the `FirstRoute` widget
onPressed: () {
  Navigator.push(
    context,
    MaterialPageRoute(builder: (context) => SecondRoute()),
  );
}

Example 2: flutter not navigating to a new screen

Wrap the new screen with a Scaffold widget

//navigation page
return GestureDetector(
  onTap: () {
    Navigator.push(
    context,
    MaterialPageRoute(
    	builder: (context) => DestinationScreen()),
  );
}

// new screen page
class _DestinationScreenState extends State<DestinationScreen> {
  @override
  Widget build(BuildContext context) {
    return Scaffold();
  }
}