flightShuttleBuilder property

Optional override to supply a widget that's shown during the hero's flight.

This in-flight widget can depend on the route transition's animation as well as the incoming and outgoing routes' Hero descendants' widgets and layout.

When both the source and destination Heros provide a flightShuttleBuilder, the destination's flightShuttleBuilder takes precedence.

If none is provided, the destination route's Hero child is shown in-flight by default.