"Invert if" should not be offered for if-case

[stereotype441]

With the following Dart 3.0 code:

f(num n) {
  if (n case int()) {
    print("it's an int");
  } else {
    print("it's not an int");
  }
}

Place the cursor on the if token and request quick assists.

Expected result: the quick assist "invert if" should not be offered.

Observed result: the quick assist "invert if" is offered, and its behaviour doesn't preserve semantics. (It exchanges the if and else blocks, but doesn't change the if statement in any other way.)

Reproduced in 60fec9c.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/297880