FormField({Key key, @required FormFieldBuilder<T> builder, FormFieldSetter<T> onSaved, FormFieldValidator<T> validator, T initialValue, bool autovalidate: false })

Source 

FormField({
  Key key,
  @required this.builder,
  this.onSaved,
  this.validator,
  this.initialValue,
  this.autovalidate: false,
}) : super(key: key) {
  assert(builder != null);
}