The name of the first route to show.
By default, this defers to dart:ui.Window.defaultRouteName.
If this string contains any
/ characters, then the string is split on
those characters and substrings from the start of the string up to each
such character are, in turn, used as routes to push.
For example, if the route
/stocks/HOOLI was used as the initialRoute,
then the Navigator would push the following routes on startup:
/stocks/HOOLI. This enables deep linking while allowing the
application to maintain a predictable route history.
final String initialRoute