Add the class value to element.
This is the Dart equivalent of jQuery's addClass.
bool add(String value) {
  _validateToken(value);
  // TODO - figure out if we need to do any validation here
  // or if the browser natively does enough.
  return modify((s) => s.add(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/add.html