countriesForTimezone method
ISO codes whose timezone list contains timezone, preserving the order
declared in zone1970.tab: the primary country (most-populous) comes
first, with any secondary countries listed after in their original order.
Returns an empty list when the timezone is unknown.
Implementation
Future<List<String>> countriesForTimezone(String timezone) async {
if (_timezoneCountries == null) await _parseZoneTab();
return _timezoneCountries![timezone] ?? const <String>[];
}