code: function joinClasses(className/*, ... */) {
    if (!className) {
      className = '';
    }
    var nextClass;
    var argLength = arguments.length;
    if (argLength > 1) {
      for (var ii = 1; ii < argLength; ii++) {
        nextClass = arguments[ii];
        if (nextClass) {
          className = (className ? className + ' ' : '') + nextClass;
        }
      }
    }
    return className;
  }
docs: Combines multiple className strings into one.
  http://jsperf.com/joinclasses-args-vs-array
  
  @param {...?string} classes
  @return {string}