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.
Intermediate routes aren't required to exist. In the example above,
/a/b could be skipped if they have no matching route. But
required to have a route, else 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