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add method
bool add(Add the class value to element.
This is the Dart equivalent of jQuery's addClass.
Source
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.dartlang.org/stable/1.24.3/dart-svg/AttributeClassSet/add.html