[vm] Incorrect nullability normalization of FutureOr in runtime type test

[askeksa-google]

import 'dart:async';

main() {
  List<Future<String?>?> l = [];
  f1(l);
  f2(l);
  print(l is List<FutureOr<String?>>);
  print(l is List<FutureOr<String?>?>);
}

void f1(List<FutureOr<String?>> l) {}
void f2(List<FutureOr<String?>?> l) {}

In the VM, this prints false for both runtime type tests, though it should print true for both.

A toString on a runtime type containing a FutureOr of a nullable type seems to never put a ? on the FutureOr type, so it looks like these types are normalized to be declared non-nullable and then erroneously treated as actually non-nullable in the subtype test.