Issue #1942490 by steveoliver, Cottser: Make Twig engine available during install.

core/includes/install.core.inc

@@ -1,6 +1,7 @@
 <?php
 
 use Drupal\Core\DrupalKernel;
+use Drupal\Core\CoreBundle;
 use Drupal\Core\Database\Database;
 use Drupal\Core\Database\Install\TaskException;
 use Drupal\Core\Language\Language;
@@ -387,6 +388,9 @@ function install_begin_request(&$install_state) {
       ->register('keyvalue.expirable.null', 'Drupal\Core\KeyValueStore\KeyValueNullExpirableFactory');
     $conf['keyvalue_expirable_default'] = 'keyvalue.expirable.null';
 
+    // Register Twig template engine for use during install.
+    CoreBundle::registerTwig($container);
+
     Drupal::setContainer($container);
   }
 

core/lib/Drupal/Core/CoreBundle.php

@@ -385,7 +385,7 @@ class CoreBundle extends Bundle {
   /**
    * Registers Twig services.
    */
-  protected function registerTwig(ContainerBuilder $container) {
+  public static function registerTwig(ContainerBuilder $container) {
     $container->register('twig.loader.filesystem', 'Twig_Loader_Filesystem')
       ->addArgument(DRUPAL_ROOT);
     $container->setAlias('twig.loader', 'twig.loader.filesystem');