getCountryTimezones method
ISO 3166-1 alpha-2 code to sorted list of IANA timezone names for that
country. Prefers zone1970.tab (which supports multi-country zones) and
falls back to zone.tab.
Implementation
Future<Map<String, List<String>>> getCountryTimezones() async {
if (_countryTimezones != null) return _countryTimezones!;
await _parseZoneTab();
return _countryTimezones!;
}