back method
The platform back gesture, injected the way the engine injects it — down
flutter/navigation — so PopScopes run on the way in.
Implementation
Future<DriveStep> back({Duration? settle}) async {
var watch = Stopwatch()..start();
await lane.popRoute();
var result = await lane.settle(settle ?? settleBudget);
return DriveStep(verb: 'back', settle: result, elapsed: watch.elapsed);
}