code: function createDom(tagName, opt_className) {
    var element = document.createElement(tagName);
    if (opt_className) {
      element.className = opt_className;
    }
    return element;
  }
docs: Creates a dom element and optionally adds a class name.
  @param {string} tagName Element's tag name.
  @param {string?} opt_className Class name to add.
  @return {Element} Created element.