getMetadata method Null safety

Implementation

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