Testing an assignment against `null` does not promote the assigned variable (unchecked_use_of_nullable_value)

```dart
int? test() => null;

void main() {
  int? foo;
  while ((foo = test()) != null) {
    foo.toDouble(); // error • The method 'toDouble' can't be unconditionally invoked because the receiver can be 'null'
  }
}
```