Неверное свойство

String? invalid

Реализация

String? get invalid native;

void invalid=(String? value)

set invalid(String? value) native;

© 2012 the Dart project authors
Licensed under the BSD 3-Clause "New" or "Revised" License.
https://api.dart.dev/stable/2.18.5/dart-html/AccessibleNode/invalid.html