The name of the first route to show, if a Navigator is built.
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.
The Navigator is only built if routes are provided (either via home,
routes, onGenerateRoute, or onUnknownRoute); if they are not,
initialRoute must be null and builder must not be null.
final String initialRoute