trackClass = (classListRef, poll, classes) => {
  classes.forEach(className => {
    incrementReference(poll, className);
    classListRef.add(className);
  });
}
Track a class and add to the given class list.
@param {Object} classListRef A class list of an element.
@param {Object} poll         The poll to be used.
@param {Array}  classes      The list of classes to be tracked.