Analyzer says unnecessary_cast, but removing cast is a compile-time error.

[lrhn]

Example:

const int minValue = (1 << 63);
T abs<T extends num>(T value) {
  if (value is int && value == minValue) return minValue as num;
  return -value;
}
main() {
  print(abs(42));
}

Here the analyzer (according to dartpad) complains that as num is unnecessary.
However, without the cast, minValue is not assignable to T.

[srawlins]

The unnecessary_cast warning now only reports when the cast type is the exact same as the expression's static type.

[lrhn]

(And eerily the following should therefore still be reported:

  List<num> nums = []; 
  if (nums is! List<int>) nums = [1] as List<int>;
  print([nums].runtimeType);

but removing the cast will change the static type of nums after the if.
You just can't be sure without knowing what the type inference would do without the cast, and the cast changes the context type of the expression it casts. This is probably good enough, the cases where an as is used for its ability to block a context type are likley few far between in any code that has to get through code review.)