RedirectResponse not working
Form redirects haven't changed between Drupal 7 and 8; you still need to use $form_state['redirect'] (incidentally drupal_goto() shouldn't be used within form submissions in Drupal 7, it can mess things up).

Within your form class's submitForm() method, you can use

$form_state['redirect'] = 'node';


Or

$form_state['redirect'] = array(
  'node/123',
  array(
    'query' =&gt; array(
      'foo' =&gt; 'bar',
    ),
    'fragment' =&gt; 'baz',
  ),
);


Full info can be found in the FormBuilder::redirectForm() docs