Determine if this element contains the class value.
This is the Dart equivalent of jQuery's hasClass.
bool contains(Object value) {
  if (value is! String) return false;
  _validateToken(value);
  return readClasses().contains(value);
}
    © 2012 the Dart project authors
Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
    https://api.dart.dev/stable/2.5.0/dart-svg/AttributeClassSet/contains.html