The application's top-level routing table.
When a named route is pushed with Navigator.pushNamed, the route name is looked up in this map. If the name is present, the associated WidgetBuilder is used to construct a PageRoute specified by pageRouteBuilder to perform an appropriate transition, including Hero animations, to the new route.
If the app only has one page, then you can specify it using home instead.
The Navigator is only built if routes are provided (either via home, routes, onGenerateRoute, or onUnknownRoute); if they are not, builder must not be null. If the routes map is not empty, the pageRouteBuilder property must be set so that the default route handler will know what kind of PageRoutes to build.
final Map<String, WidgetBuilder> routes