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;
}
Combines multiple className strings into one.
http://jsperf.com/joinclasses-args-vs-array

@param {...?string} className
@return {string}