Generate nested routes

The setup flow’s job is to display the appropriate
page within the flow.

Add a Navigator widget to SetupFlow,
and implement the onGenerateRoute property.

The _onGenerateRoute function works the same as
for a top-level Navigator. A RouteSettings
object is passed into the function,
which includes the route’s name.
Based on that route name,
one of four flow pages is returned.

The finished page provides the user with a Finish
button. When the user taps Finish,
the _exitSetup callback is invoked, which pops the entire 
setup flow off the top-level Navigator stack,
taking the user back to the home screen.

Congratulations!
You implemented nested navigation with four subroutes.

Interactive example

Run the app: