link.html.twig provided with indexed links will no longer produce '_' classes

Providing indexed instead of associative array of class would produce an unexpected class="_" in markup of links.html.twig. Resolved this by moving the class generation to Stable's preprocess.

Renderable array for links.html.twig

Indexed array

$build['links'] = [
  '#theme' => 'links',
  '#links' => [
    ['title' => 'Block', 'url' => '/admin/help/block'], 
    ['title' => 'Breakpoint', 'url' => '/admin/help/breakpoint'],
  ];

Associative array

$build['links'] = [
  '#theme' => 'links',
  '#links' => [
    'block' => ['title' => 'Block', 'url' => '/admin/help/block'], 
    'breakpoint' =>['title' => 'Breakpoint', 'url' => '/admin/help/breakpoint'],
  ];

Before

Indexed array

<ul class="links">
  <li class="_"><a href="/admin/help/block">Block</a></li>
  <li class="_"><a href="/admin/help/breakpoint">Breakpoint</a></li>
</ul>

Associative array

<ul class="links">
  <li class="block"><a href="/admin/help/block">Block</a></li>
  <li class="breakpoint"><a href="/admin/help/breakpoint">Breakpoint</a></li>
</ul>

After

Indexed array

<ul class="links">
  <li><a href="/admin/help/block">Block</a></li>
  <li><a href="/admin/help/breakpoint">Breakpoint</a></li>
</ul>

Associative array

<ul class="links">
  <li><a href="/admin/help/block">Block</a></li>
  <li><a href="/admin/help/breakpoint">Breakpoint</a></li>
</ul>

Regarding Stark/Core and users of base theme: false

If you are using base theme: false. And need this feature, please copy the preprocess theme hook from stable into your theme.

/**
 * Implements template_preprocess_links().
 */
function THEME_preprocess_links(&$variables) {
  if (!empty($variables['links'])) {
    foreach ($variables['links'] as $key => $value) {
      if (!is_numeric($key)) {
        $class = Html::getClass($key);
        $variables['links'][$key]['attributes']->addClass($class);
      }
    }
  }
}