I'm trying to change a label of Drupal 8's contact form ('Your email address'). The label is defined /core/modules/contact/src/MessageForm.php, but obviously I don't want to change this in core.
$form['mail'] = array( '#type' => 'email', '#title' => $this->t('Your email address'), '#required' => TRUE, );
In Drupal 7 I always used string overrides but the module isn't available for Drupal 8 yet. How can string be overridden in Drupal 8?