readEntries method Null safety

Implementation

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

  return completer.future;
}