How do I invalidate a page from cache? In Drupal 7 we could clear a cached URL with the following.

$url = url('&lt;front&gt;',  array('absolute' =&gt; TRUE));
cache_clear_all($url, 'cache_page');


What's the equivalent in Drupal 8?
There is no equivalent in D8. What would be the closest to this is to assign a custom cache tag to the page and then invalidate it. See https://www.drupal.org/docs/8/api/cache-api/cache-tags.

But normally you don't need to invalidate cache tags in custom code. If you set the correct cache metadata this should happen automatically.