code: function protectedFactory(instance) {
    var publicPrototype = Object.getPrototypeOf(instance);
    var protectedPrototype = protectToPrototypeMap.get(publicPrototype);
    if (!protectedPrototype) {
      throw new Error('The protected key function only accepts instances '
        + 'of objects created using Mozart constructors.'
      );
    }
    return Object.create(protectedPrototype);
  }
docs: A creator function that can be passed to the Private Parts `createKey`
  method. If accepts an object and return a new object whose prototype is
  the protected counterpart to the prototype of the passed object.
  @param {Object} instance An instance of a constructor created with Class.
  @return {Object} The protected methods object.