nullable value fails to narrow when used with ?? operator

[DetachHead]

void foo(Map<String, Object>? value) {
  if (value?.keys.contains('') ?? false) {
    value.keys; // Error: Property 'keys' cannot be accessed on 'Map<String, Object>?' because it is potentially null.
  }
}

[eernstg]

Note that this is similar to dart-lang/language#1224, and this particular idea is mentioned in this comment.