## Description

This PR is a reland of https://github.com/flutter/flutter/pull/129274 with a fix and new test related to the revert (https://github.com/flutter/flutter/pull/131303).

It 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).

![image](https://github.com/flutter/flutter/assets/840911/08fcee6c-b286-4749-ad0b-ba09e653bd94)

## 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 2 tests.