[analyzer] Implement FutureOr related rules of least upper bound

[eernstg]

This is the analyzer specific issue for #43719, which has the details.

[scheglov]

The test below fails because UP(FutureOr<void>, dynamic) is dynamic. We apply UP(T1, T2) where TOP(T1) and TOP(T2) and MORETOP(FutureOr<void>, dynamic) is false, so the result is T2 which is dynamic.

  void f20(dynamic a, FutureOr<void> b) {
    var x = condition ? a : b;
    x.expectStaticType<Exactly<FutureOr<void>>>();
    if (x is Future) throw 0;
    /**/ x.toString();
    //   ^
    // [analyzer] COMPILE_TIME_ERROR.USE_OF_VOID_RESULT
    // [cfe] This expression has type 'void' and can't be used.

    var y = condition ? b : a;
    y.expectStaticType<Exactly<FutureOr<void>>>();
    if (y is Future) throw 0;
    /**/ y.toString();
    //   ^
    // [analyzer] COMPILE_TIME_ERROR.USE_OF_VOID_RESULT
    // [cfe] This expression has type 'void' and can't be used.
  }

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/166786

[eernstg]

@scheglov wrote:

.. the result is T2 which is dynamic.

Indeed, thanks for catching that! Fixed in a369992.