How do you implement a custom submit handler in hook_form_alter()? I've been updating one of my Drupal 7 modules to 8 and I'm having an issue with my form alter. I'm adding a validation function and a submit handler. The validation function is getting called just fine, but the submit function is not getting fired. I'm wondering if this means there is a Symfony-style approach that needs to be taken to adding submit handlers to existing forms. I've done a lot of searching and I can't find any examples. Also looked through some work-in-progress D8 modules but I haven't really found any working examples there either.

Does anyone have any idea what the correct Drupal 8 way to do this is?
According to the example in the documentation it's the same as for Drupal 7:

$form['actions']['submit']['#submit'][] = 'mymodule_upload_enabled_types_submit';


You can find similar (working) logic in 


contact_form_user_form_alter()
editor_form_filter_format_form_alter()
locale_form_language_admin_edit_form_alter()


And a few others besides.