diff -u b/core/includes/install.inc b/core/includes/install.inc
--- b/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -556,7 +556,7 @@
   $info = $install_state['profile_info'];
 
   // Get the list of available modules for the selected installation profile.
-  $listing = new ExtensionDiscovery(DRUPAL_ROOT);
+  $listing = new ExtensionDiscovery(\Drupal::root());
   $present_modules = array();
   foreach ($listing->scan('module') as $present_module) {
     $present_modules[] = $present_module->getName();
@@ -923,10 +923,10 @@
   // Collect requirement testing results.
   $requirements = array();
 
-  $listing = new ExtensionDiscovery(DRUPAL_ROOT);
+  $listing = new ExtensionDiscovery(\Drupal::root());
   $module_list = $listing->scan('module');
   foreach ($info['dependencies'] as $module) {
-    $file = DRUPAL_ROOT . '/' . $module_list[$module]->getPath() . "/$module.install";
+    $file = \Drupal::root() . '/' . $module_list[$module]->getPath() . "/$module.install";
     if (is_file($file)) {
       require_once $file;
     }
diff -u b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
--- b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
+++ b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
@@ -85,7 +85,7 @@
 
       $install_profile = Settings::get('install_profile');
       $extensions = $this->configStorage->read('core.extension');
-      $listing = new ExtensionDiscovery(DRUPAL_ROOT);
+      $listing = new ExtensionDiscovery(\Drupal::root());
       if (!empty($extensions['module'])) {
         $modules = $extensions['module'];
         // Remove the install profile as this is handled later.
diff -u b/core/lib/Drupal/Core/Config/InstallStorage.php b/core/lib/Drupal/Core/Config/InstallStorage.php
--- b/core/lib/Drupal/Core/Config/InstallStorage.php
+++ b/core/lib/Drupal/Core/Config/InstallStorage.php
@@ -159,7 +159,7 @@
     if (!isset($this->folders)) {
       $this->folders = array();
       $this->folders += $this->getCoreNames();
-      $listing = new ExtensionDiscovery(DRUPAL_ROOT);
+      $listing = new ExtensionDiscovery(\Drupal::root());
       if ($profile = drupal_get_profile()) {
         $profile_list = $listing->scan('profile');
         if (isset($profile_list[$profile])) {
@@ -185,9 +185,11 @@
     $extension = '.' . $this->getFileExtension();
     $folders = array();
     foreach ($list as $extension_object) {
+      // We don't have to use ExtensionDiscovery here because our list of
+      // extensions was already obtained through an ExtensionDiscovery scan.
       $directory = $this->getComponentFolder($extension_object);
       if (file_exists($directory)) {
-        $files = new \GlobIterator(DRUPAL_ROOT . '/' . $directory . '/*' . $extension);
+        $files = new \GlobIterator(\Drupal::root() . '/' . $directory . '/*' . $extension);
         foreach ($files as $file) {
           $folders[$file->getBasename($extension)] = $directory;
         }
@@ -207,7 +209,7 @@
     $folders = array();
     $directory = $this->getCoreFolder();
     if (file_exists($directory)) {
-      $files = new \GlobIterator(DRUPAL_ROOT . '/' . $directory . '/*' . $extension);
+      $files = new \GlobIterator(\Drupal::root() . '/' . $directory . '/*' . $extension);
       foreach ($files as $file) {
         $folders[$file->getBasename($extension)] = $directory;
       }
diff -u b/core/modules/simpletest/src/KernelTestBase.php b/core/modules/simpletest/src/KernelTestBase.php
--- b/core/modules/simpletest/src/KernelTestBase.php
+++ b/core/modules/simpletest/src/KernelTestBase.php
@@ -484,7 +484,7 @@
    *   The new modules are only added to the active module list and loaded.
    */
   protected function enableModules(array $modules) {
-    $listing = new ExtensionDiscovery(DRUPAL_ROOT);
+    $listing = new ExtensionDiscovery(\Drupal::root());
     $module_list = $listing->scan('module');
     // In ModuleHandlerTest we pass in a profile as if it were a module.
     $module_list += $listing->scan('profile');