loadFile method
Load the fileName using one of the strategies provided
Implementation
@protected
Future<Map> loadFile(final String fileName) async {
final List<Future<Map?>> strategiesFutures = _executeStrategies(fileName);
final Stream<Map?> strategiesStream = Stream.fromFutures(strategiesFutures);
return await strategiesStream.firstWhere((map) => map != null,
orElse: null) ??
Map();
}