diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 968192e..0bf5a97 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -73,6 +73,10 @@
  * an error is thrown, (b) a new page needs to be displayed, or (c) the
  * installation finishes (whichever happens first).
  *
+ * @param $class_loader
+ *   The class loader. Normally Composer's ClassLoader, as included by the
+ *   front controller, but may also be decorated; e.g.,
+ *   \Symfony\Component\ClassLoader\ApcClassLoader.
  * @param $settings
  *   An optional array of installation settings. Leave this empty for a normal,
  *   interactive, browser-based installation intended to occur over multiple
@@ -84,7 +88,7 @@
  *
  * @see install_state_defaults()
  */
-function install_drupal($settings = array()) {
+function install_drupal($class_loader, $settings = array()) {
   global $install_state;
   // Initialize the installation state with the settings that were passed in,
   // as well as a boolean indicating whether or not this is an interactive
@@ -95,7 +99,7 @@ function install_drupal($settings = array()) {
   try {
     // Begin the page request. This adds information about the current state of
     // the Drupal installation to the passed-in array.
-    install_begin_request($install_state);
+    install_begin_request($class_loader, $install_state);
     // Based on the installation state, run the remaining tasks for this page
     // request, and collect any output.
     $output = install_run_tasks($install_state);
@@ -252,11 +256,15 @@ function install_state_defaults() {
  * This function performs commands that must run at the beginning of every page
  * request. It throws an exception if the installation should not proceed.
  *
+ * @param $class_loader
+ *   The class loader. Normally Composer's ClassLoader, as included by the
+ *   front controller, but may also be decorated; e.g.,
+ *   \Symfony\Component\ClassLoader\ApcClassLoader.
  * @param $install_state
  *   An array of information about the current installation state. This is
  *   modified with information gleaned from the beginning of the page request.
  */
-function install_begin_request(&$install_state) {
+function install_begin_request($class_loader, &$install_state) {
   $request = Request::createFromGlobals();
 
   // Add any installation parameters passed in via the URL.
@@ -286,7 +294,6 @@ function install_begin_request(&$install_state) {
   }
 
   $site_path = DrupalKernel::findSitePath($request, FALSE);
-  $class_loader = require __DIR__ . '/../vendor/autoload.php';
   Settings::initialize(dirname(dirname(__DIR__)), $site_path, $class_loader);
 
   // Ensure that procedural dependencies are loaded as early as possible,
diff --git a/core/install.php b/core/install.php
index e206dfb..3493d4a 100644
--- a/core/install.php
+++ b/core/install.php
@@ -27,6 +27,6 @@
 }
 
 // Start the installer.
-require_once __DIR__ . '/vendor/autoload.php';
+$class_loader = require_once __DIR__ . '/vendor/autoload.php';
 require_once __DIR__ . '/includes/install.core.inc';
-install_drupal();
+install_drupal($class_loader);
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 93a5047..c451574 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -842,7 +842,7 @@ protected function setUp() {
 
     // Execute the non-interactive installer.
     require_once DRUPAL_ROOT . '/core/includes/install.core.inc';
-    install_drupal($parameters);
+    install_drupal($class_loader, $parameters);
 
     // Import new settings.php written by the installer.
     Settings::initialize(DRUPAL_ROOT, $directory, $class_loader);