False-positive unnecessary_cast when making a type nullable.

[naipofo]

Describe the issue
Linter gives unnecessary_cast despite the cast fixing a type error.

To Reproduce

void main() {
  final list = [1,2,3,4];

  // type error: The return type 'Null' isn't a 'int', as required by the closure's context.
  final maybeFound1 = list.firstWhere((e) => e == 5, orElse: () => null);

  // error fixed but lint says that there is an unnecessary cast
  final maybeFound2 = (list as List<int?>).firstWhere((e) => e == 5, orElse: () => null);
}

Expected behavior
No lint message, as the cast is not unnecessary.

Additional context
Applying the lint fix as suggested causes a type error and dart refuses to compile.

[srawlins]

More or less a duplicate of #49550 (P2), and broadly #52086 (P3). See also #52117, #49269, #48984, #34150. This can be fixed by https://dart-review.googlesource.com/c/sdk/+/265420.

[srawlins]

Fixed with the fix to those issues.