The JsAllowValueClassesInExternals feature is enabled explicitly,
because otherwise it's enabled
implicitly depending on the backend. See:
org/jetbrains/kotlin/test/builders/LanguageVersionSettingsBuilder.kt:90
A property may have a fake source return kind, while its accessor
has a real source kind. In this case we can't "just copy"
the property return type down to the accessor.
Artem Kobzar