Two tests in LanguageFeatures/Private-fields-promotion expect to be able to type promote fields of a static value

[stereotype441]

The tests LanguageFeatures/Private-fields-promotion/promotion_A01_t07 and LanguageFeatures/Private-fields-promotion/promotion_A01_t08 both contain code that looks roughly like this:

enum E {
  e1, e2;

  final int? _x = 42;
}

main() {
  if (E.e1._x != null) {
    E.e1._x.isEven;
  }
}

For Dart 3.2, field promotion only considers an expression of the form target._fieldName to be stable (and thus promotable) if _fieldName represents a promotable field and target is one of the following:

• this (includes implicit uses of this, so bare _fieldName is considered promotable inside an instance method of a class)
• super
• A local variable that isn't modified or write captured during the time period covered by the potential promotion.
• An expression of the form target._fieldName that's considered stable.

We have considered expanding this list to include static constant fields such as E.e1 (dart-lang/language#3326), but we haven't done so; if we do decide to make this change, we won't make it until a future Dart release.

Accordingly, these two tests need to be updated so that they don't expect expressions like E.e1._x to undergo type promotion.