The name of the first route to show.
Defaults to Window.defaultRouteName, which may be overridden by the code that launched the application.
If the route contains slashes, then it is treated as a "deep link", and
before this route is pushed, the routes leading to this one are pushed
also. For example, if the route was
/a/b/c, then the app would start
with the three routes
/a/b/c loaded, in that order.
If any part of this process fails to generate routes, then the
initialRoute is ignored and Navigator.defaultRouteName is used instead
/). This can happen if the app is started with an intent that specifies
a non-existent route.
final String initialRoute