implementation.dart

const RefreshIndicator({
  Key key,
  @required this.child,
  this.displacement = 40.0,
  @required this.onRefresh,
  this.color,
  this.backgroundColor,
  this.notificationPredicate = defaultScrollNotificationPredicate,
  this.semanticsLabel,
  this.semanticsValue,
}) : assert(child != null),
     assert(onRefresh != null),
     assert(notificationPredicate != null),
     super(key: key);