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contains method
bool contains(Determine if this element contains the class value.
This is the Dart equivalent of jQuery's hasClass.
Source
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.dartlang.org/stable/1.24.3/dart-svg/AttributeClassSet/contains.html