Flow analysis. An assignment of null promotes to non-nullable type

[sgrekhov]

The code below works without errors in either CFE and the analyzer.

test1(Object? x) {
  switch (x) {
    case null:
      x.expectStaticType<Exactly<Object?>>(); // Ok
    case _:
      x.expectStaticType<Exactly<Object>>();
  }
}

test2(Object? x) {
  switch (x) {
    case null when x = null == null:
      x.expectStaticType<Exactly<Object>>(); /// Seems that assignment `x = null` promotes `x` from `Object?` to `Object`
    case _:
      x.expectStaticType<Exactly<Object>>();
  }
}

main() {
  test1(1);
  test1(null);
  test2(1);
  test2(null);
}

extension StaticType<T> on T {
  T expectStaticType<R extends Exactly<T>>() {
    return this;
  }
}
typedef Exactly<T> = T Function(T);

cc @stereotype441

Dart SDK version: 3.9.0-74.0.dev (dev) (Tue Apr 29 21:02:55 2025 -0700) on "windows_x64"