## Description
This PR updates how a floating snack bar is positionned when a `Scaffold` defines a FAB with `Scaffold.floatingActionButtonLocation` sets to one of the top locations.
**Before this PR:**
- When a FAB location is set to the top of the `Scaffold`, a floating `SnackBar` can't be displayed and an assert throws in debug mode.
**After this PR:**
- When a FAB location is set to the top of the `Scaffold`, a floating `SnackBar` will be displayed at the bottom of the screen, above a `NavigationBar` for instance (the top FAB is ignored when computing the floating snack bar position).

## Motivation
This is a edge case related to a discrepancy between the Material spec and the Flutter `Scaffold` customizability:
- Material spec states that a floating `SnackBar` should be displayed above a FAB. But, in Material spec, FABs are expected to be on the bottom.
- Since https://github.com/flutter/flutter/issues/51465, Flutter `Scaffold` makes it valid to show a FAB on the top of the `Scaffold`.
## Related Issue
fixes https://github.com/flutter/flutter/issues/128150
## Tests
Adds 1 test.