Null-aware return doesn't promote variable

Consider this code:

```dart
final lastTransaction = transactions.lastOrNull;
if (lastTransaction?.expirationDate == null) {
return false;
}

final expirationDate = DateTime.tryParse(lastTransaction!.expirationDate!);
```

If I remove the `!`, I get an analyser error saying I can't use `lastTransaction` because it can be `null`. I believe the analyser should be able to conclude that `lastTransaction` can't be `null`, because it's a final local variable, and the `?.` in the `if` check would ensure there's no valid code path for it to still be `null`.

Start by reproducing the reported Dart snippet and checking the analyzer's null-safety promotion behavior after the null-aware condition. Read the language's rules for local-variable promotion and null-aware access; done means the analyzer consistently accepts using lastTransaction and expirationDate without the non-null assertions, with coverage for this example.

Written by the indexing model from the issue text.