How to use the class-utils.extend function in class-utils

* Parent.extend(Child);
   *
   * // optional methods
   * Parent.extend(Child, {
   *   foo: function() {},
   *   bar: function() {}
   * });
   * ```
   * @name #extend
   * @param {Function} `Ctor` constructor to extend
   * @param {Object} `methods` Optional prototype properties to mix in.
   * @return {Object} Returns the `Base` constructor for chaining
   * @api public
   */

  define(Base, 'extend', cu.extend(Base, function(Ctor, Parent) {
    Ctor.prototype.mixins = Ctor.prototype.mixins || [];

    define(Ctor, 'mixin', function(fn) {
      var mixin = fn(Ctor.prototype, Ctor);
      if (typeof mixin === 'function') {
        Ctor.prototype.mixins.push(mixin);
      }
      return Ctor;
    });

    define(Ctor, 'mixins', function(Child) {
      Base.run(Child, 'mixin', Ctor.prototype.mixins);
      return Ctor;
    });

    Ctor.prototype.mixin = function(key, value) {