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 MaterialPageRoute that performs
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.
If home is specified, then it implies an entry in this table for the
Navigator.defaultRouteName route (
/), and it is an error to
redundantly provide such a route in the routes table.
If a route is requested that is not specified in this table (or by
home), then the onGenerateRoute callback is called to build the page
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.
final Map<String, WidgetBuilder> routes