getParent method Null safety

Implementation

@JSName('getParent')
Future<Entry> getParent() {
  var completer = new Completer<Entry>();
  _getParent((value) {
    applyExtension('Entry', value);
    completer.complete(value);
  }, (error) {
    completer.completeError(error);
  });
  return completer.future;
}