Function drupal_goto() is gone and must be replaced with the use of something else.
For example, in AddUpdateForm::submitForm(), which is a method implemented in a class derived from FormBase, I changed
$form_state['redirect_route']['route_name'] =where optimizely.listing is a route defined in the routing file and $form_state is a parameter passed in by reference.
The source article below suggests the use of RedirectResponse() instead of drupal_goto(). However, I haven't been able to figure out how to use it in submitForm(), and I've yet to find examples that are at my level of understanding.
Finally, here's a quote from a relevant comment made by Crell, a.k.a. Larry Garfield:
"For instance, if you're using the common controller base class there is a $this->redirect($route_name) method ready for you; ... "That looks very straightforward, but the required context is being in an instance of the common controller base class.
Update: redirection can no longer be done as above. See the note on method setRedirect() in this newer post http://optimizely-to-drupal-8.blogspot.com/2014/08/drupal-8-alpha-13-alpha-14.html#setredirect
drupal_goto() has been removed