diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -152,6 +152,9 @@
       // Redirect to the correct page if the URL parameters have changed.
       install_goto(install_redirect_url($state));
     }
+    elseif ($output instanceof Response) {
+      $output->send();
+    }
     elseif (isset($output)) {
       // Display a page only if some output is available. Otherwise it is
       // possible that we are printing a JSON page and theme output should
@@ -671,14 +674,8 @@
     // any output from the batch process, until the task is complete.
     elseif ($current_batch == $function) {
       $output = _batch_page(\Drupal::request());
-      // Because Batch API now returns a JSON response for intermediary steps,
-      // but the installer doesn't handle Response objects yet, just send the
-      // output here and emulate the old model.
-      // @todo Replace this when we refactor the installer to use a request-
-      //   response workflow.
       if ($output instanceof Response) {
-        $output->send();
-        $output = NULL;
+        return $output;
       }
       // The task is complete when we try to access the batch page and receive
       // FALSE in return, since this means we are at a URL where we are no
@@ -1547,8 +1544,6 @@
   $session = \Drupal::service('session');
   \Drupal::request()->setSession($session);
   $session->start();
-  // Ensure the session is maintained throughout the install.
-  $session->set('_drupal_installing', TRUE);
 }
 
 /**
@@ -1846,9 +1841,6 @@
   // Installation profiles are always loaded last.
   module_set_weight($profile, 1000);
 
-  // Clean up the session flag.
-  \Drupal::service('session')->remove('_drupal_installing');
-
   // Build the router once after installing all modules.
   // This would normally happen upon KernelEvents::TERMINATE, but since the
   // installer does not use an HttpKernel, that event is never triggered.