code: function classes(input) {
    if (!input) return [];
    assert.strictEqual(typeof input, 'string', 'expected a string for the class name');
    if (!input.trim()) return [];
    return input.trim().split(/\s+/g);
  }
docs: private helpers 
  Parse the given `input` into an `Array` of class names. Will always return
  an `Array`, even if it's empty.
  
  @param {String} [input]  The class attribute string.
  @return {Array}