leading property
The leading slot: the automatic back button, the declared leading items, their placeholders, or null where the route has neither.
A single widget rather than a list, so it drops into AppBar.leading and
GlassAppBar.leading unchanged; where a back button and leading items
both show, it is the Row holding the two.
Always null in iPhone Duo's vertical bar strip (GlassVerticalBar.maybeOf), where the back button and every item that goes vertical leave the bar for the strip.
Implementation
final Widget? leading;