code: function getClassIndex(element, className) {
    assertType(element, Node, false, 'Invalid element specified');
    assertType(className, 'string', false, `Invalid class name: ${className}`);
    let classList = element.className.split(' ');
    return classList.indexOf(className);
  }
docs: Gets the index of a specified class in a DOM element,
  
  @param {Node} element - Target element.
  @param {string} className - Target class name.
  
  @return {number} Index of given class name. -1 if not found.
  
  @alias module:requiem~dom.getClassIndex