The region subtag for the locale.
This may be null, indicating that there is no specified region subtag.
This is expected to be string registered in the IANA Language Subtag Registry with the type "region". The string specified must match the case of the string in the registry.
Region subtags that are deprecated in the registry and have a preferred
code are changed to their preferred code. For example,
const Locale('de', 'DE') and
const Locale('de', 'DD') are equal, and both have the
DD is a deprecated language subtag that was
replaced by the subtag
String get countryCode => _deprecatedRegionSubtagMap[_countryCode] ?? _countryCode;