readEntries method Null safety

Implementation

Future<List<Entry>> readEntries() {
  var completer = new Completer<List<Entry>>();
  _readEntries((values) {
    values.forEach((value) {
      applyExtension('Entry', value);
      Entry entry = value as Entry;
      if (entry.isFile)
        applyExtension('FileEntry', entry);
      else if (entry.isDirectory) applyExtension('DirectoryEntry', entry);
    });
    completer.complete(new List<Entry>.from(values));
  }, (error) {
    completer.completeError(error);
  });

  return completer.future;
}