D8: Add a custom submission handler to a form

demo.php

<?php

use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\HttpFoundation\Request;

function MYMODULE_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'node_trends_form' || $form_id == 'node_trends_edit_form') {
    foreach (array_keys($form['actions']) as $action) {
      if ($action != 'preview' && isset($form['actions'][$action]['#type']) && $form['actions'][$action]['#type'] === 'submit') {
        $form['actions']['submit']['#submit'][] = 'mymodule_form_submit';
      }
    }
  }
}

function mymodule_form_submit(array $form, FormStateInterface $form_state){
    //die("why won't this execute? :(");
    drupal_set_message("Why won't this message show?");
}