CFE: Accept super-bounded type also when top type uses FutureOr

[eernstg]

Consider the following program:

import 'dart:async';
class C<X extends num> {}

void main() {
  C<FutureOr<Object?>> x;
}

This program is rejected by dart (eb4cf54) as follows:

n021.dart:5:24: Error: Type argument 'FutureOr<Object?>' doesn't conform to the bound 'num' of the type variable 'X' on 'C'.
 - 'Object' is from 'dart:core'.
Try changing type arguments so that they conform to the bounds.
  C<FutureOr<Object?>> x;
                       ^
n021.dart:2:9: Context: This is the type variable whose bound isn't conformed to.
class C<X extends num> {}
        ^

However, C<FutureOr<Object?>> is correctly super-bounded because FutureOr<Object?> is a top type. The error seems to occur only with FutureOr, so it's probably a matter of checking for all top types rather than just some of them when it is considered whether this could be a super-bounded type.