[linter request] Don't use a non-nullable casting type when combined with conditional member access

[Reprevise]

Dart should have a lint to ensure that users who combine conditional access and casting that the casting type is nullable. An alternative solution would be to have the user switch to using the bang (!) operator instead to show intent.

BAD:

bool? getBool(List<List<Object>?> input) {
  return input.firstOrNull?.single as bool; // Non-null type combined with conditional access, will throw a TypeError
}

void main() {
  final objects = <List<Object>?>[null, ["bar"], [2]];
  getBool(objects);
}

GOOD:

bool? getBool(List<List<Object>?> input) {
  return input.firstOrNull?.single as bool?; // Nullable type combined with conditional access, no TypeError
}

bool? getBool(List<List<Object>?> input) {
  return input.firstOrNull!.single as bool; // Using a bang operator instead
}

void main() {
  final objects = <List<Object>?>[null, ["bar"], [2]];
  getBool(objects);
}

[FMorschel]

As mentioned on #59513 (somewhat related), this is somewhat related to dart-lang/language#4074.