diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index eeacd7a..e8808c2 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -249,16 +249,9 @@ function drupal_get_filename($type, $name, $filename = NULL) {
     if (!isset($files[$type][$name]) && \Drupal::hasService('state')) {
       $files[$type] += \Drupal::state()->get('system.' . $type . '.files', array());
     }
-    // If still unknown, perform a filesystem scan.
+    // If still unknown, create a user-level error message.
     if (!isset($files[$type][$name])) {
-      $listing = new ExtensionDiscovery(DRUPAL_ROOT);
-      // Prevent an infinite recursion by this legacy function.
-      if ($original_type == 'profile') {
-        $listing->setProfileDirectories(array());
-      }
-      foreach ($listing->scan($original_type) as $extension_name => $file) {
-        $files[$type][$extension_name] = $file->getPathname();
-      }
+      trigger_error(SafeMarkup::format('The following @type is missing from the file system: @name', array('@type' => $type, '@name' => $name)), E_USER_WARNING);
     }
   }
 
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index d858875..dd1d5e9 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -302,6 +302,10 @@ function install_begin_request($class_loader, &$install_state) {
   // Allow command line scripts to override server variables used by Drupal.
   require_once __DIR__ . '/bootstrap.inc';
 
+  // Prime the drupal_get_filename() cache with the location of the system
+  // module.
+  drupal_get_filename('module', 'system', 'core/modules/system/system.info.yml');
+
   // If the hash salt leaks, it becomes possible to forge a valid testing user
   // agent, install a new copy of Drupal, and take over the original site.
   // The user agent header is used to pass a database prefix in the request when
@@ -1031,6 +1035,7 @@ function install_base_system(&$install_state) {
 
   // Enable the user module so that sessions can be recorded during the
   // upcoming bootstrap step.
+  system_rebuild_module_data();
   \Drupal::service('module_installer')->install(array('user'), FALSE);
 
   // Save the list of other modules to install for the upcoming tasks.
diff --git a/core/includes/install.inc b/core/includes/install.inc
index 7e0e32e..604c82d 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -922,11 +922,19 @@ function drupal_check_profile($profile, array $install_state) {
 
   // Collect requirement testing results.
   $requirements = array();
+
+  $listing = new ExtensionDiscovery(DRUPAL_ROOT);
+  $module_list = $listing->scan('module');
+  // Make sure the installation API is available
+  include_once __DIR__ . '/install.inc';
   foreach ($info['dependencies'] as $module) {
-    module_load_install($module);
+    $file = DRUPAL_ROOT . '/' . $module_list[$module]->getPath() . "/$module.install";
+    if (is_file($file)) {
+      require_once $file;
+    }
     $function = $module . '_requirements';
 
-    drupal_classloader_register($module, drupal_get_path('module', $module));
+    drupal_classloader_register($module, $module_list[$module]->getPath());
     if (function_exists($function)) {
       $requirements = array_merge($requirements, $function('install'));
     }
diff --git a/core/lib/Drupal/Core/Config/ConfigInstaller.php b/core/lib/Drupal/Core/Config/ConfigInstaller.php
index c109ea7..d87c142 100644
--- a/core/lib/Drupal/Core/Config/ConfigInstaller.php
+++ b/core/lib/Drupal/Core/Config/ConfigInstaller.php
@@ -157,17 +157,23 @@ public function installDefaultConfig($type, $name) {
    * {@inheritdoc}
    */
   public function installOptionalConfig(StorageInterface $storage = NULL, $dependency = []) {
+    $profile = $this->drupalGetProfile();
     if (!$storage) {
       // Search the install profile's optional configuration too.
       $storage = new ExtensionInstallStorage($this->getActiveStorages(StorageInterface::DEFAULT_COLLECTION), InstallStorage::CONFIG_OPTIONAL_DIRECTORY, StorageInterface::DEFAULT_COLLECTION, TRUE);
       // The extension install storage ensures that overrides are used.
       $profile_storage = NULL;
     }
-    else {
+    elseif (isset($profile)) {
       // Creates a profile storage to search for overrides.
-      $profile_install_path = $this->drupalGetPath('module', $this->drupalGetProfile()) . '/' . InstallStorage::CONFIG_OPTIONAL_DIRECTORY;
+      $profile_install_path = $this->drupalGetPath('module', $profile) . '/' . InstallStorage::CONFIG_OPTIONAL_DIRECTORY;
       $profile_storage = new FileStorage($profile_install_path, StorageInterface::DEFAULT_COLLECTION);
     }
+    else {
+      // Profile has not been set yet. For example during the first steps of the
+      // installer or during unit tests.
+      $profile_storage = NULL;
+    }
 
     $collection_info = $this->configManager->getConfigCollectionInfo();
     $enabled_extensions = $this->getEnabledExtensions();
diff --git a/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
index 15490b4..b75ef3c 100644
--- a/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
+++ b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Config;
 
 use Drupal\Core\Site\Settings;
+use Drupal\Core\Extension\ExtensionDiscovery;
 
 /**
  * Storage to access configuration and schema in enabled extensions.
@@ -80,26 +81,40 @@ public function createCollection($collection) {
   protected function getAllFolders() {
     if (!isset($this->folders)) {
       $this->folders = array();
-      $this->folders += $this->getComponentNames('core', array('core'));
+      $this->folders += $this->getCoreNames();
 
       $install_profile = Settings::get('install_profile');
       $extensions = $this->configStorage->read('core.extension');
+      $listing = new ExtensionDiscovery(DRUPAL_ROOT);
       if (!empty($extensions['module'])) {
         $modules = $extensions['module'];
         // Remove the install profile as this is handled later.
         unset($modules[$install_profile]);
-        $this->folders += $this->getComponentNames('module', array_keys($modules));
+        $module_list_scan = $listing->scan('module');
+        $module_list = array();
+        foreach (array_keys($modules) as $module) {
+          $module_list[$module] = $module_list_scan[$module];
+        }
+        $this->folders += $this->getComponentNames($module_list);
       }
       if (!empty($extensions['theme'])) {
-        $this->folders += $this->getComponentNames('theme', array_keys($extensions['theme']));
+        $theme_list_scan = $listing->scan('theme');
+        foreach (array_keys($extensions['theme']) as $theme) {
+          $theme_list[$theme] = $theme_list_scan[$theme];
+        }
+        $this->folders += $this->getComponentNames($theme_list);
       }
 
       if ($this->includeProfile) {
         // The install profile can override module default configuration. We do
         // this by replacing the config file path from the module/theme with the
         // install profile version if there are any duplicates.
-        $profile_folders = $this->getComponentNames('profile', array(drupal_get_profile()));
-        $this->folders = $profile_folders + $this->folders;
+        $profile = drupal_get_profile();
+        if (isset($profile)) {
+          $profile_list = $listing->scan('profile');
+          $profile_folders = $this->getComponentNames(array($profile_list[$profile]));
+          $this->folders = $profile_folders + $this->folders;
+        }
       }
     }
     return $this->folders;
diff --git a/core/lib/Drupal/Core/Config/InstallStorage.php b/core/lib/Drupal/Core/Config/InstallStorage.php
index 06618d7..800b668 100644
--- a/core/lib/Drupal/Core/Config/InstallStorage.php
+++ b/core/lib/Drupal/Core/Config/InstallStorage.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Config;
 
 use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Extension\Extension;
 
 /**
  * Storage used by the Drupal installer.
@@ -157,14 +158,14 @@ public function listAll($prefix = '') {
   protected function getAllFolders() {
     if (!isset($this->folders)) {
       $this->folders = array();
-      $this->folders += $this->getComponentNames('core', array('core'));
-      // @todo Refactor getComponentNames() to use the extension list directly.
+      $this->folders += $this->getCoreNames();
+      $listing = new ExtensionDiscovery(DRUPAL_ROOT);
       if ($profile = drupal_get_profile()) {
-        $this->folders += $this->getComponentNames('profile', array($profile));
+        $profile_list = $listing->scan('profile');
+        $this->folders += $this->getComponentNames(array($profile_list[$profile]));
       }
-      $listing = new ExtensionDiscovery(DRUPAL_ROOT);
-      $this->folders += $this->getComponentNames('module', array_keys($listing->scan('module')));
-      $this->folders += $this->getComponentNames('theme', array_keys($listing->scan('theme')));
+      $this->folders += $this->getComponentNames($listing->scan('module'));
+      $this->folders += $this->getComponentNames($listing->scan('theme'));
     }
     return $this->folders;
   }
@@ -172,19 +173,17 @@ protected function getAllFolders() {
   /**
    * Get all configuration names and folders for a list of modules or themes.
    *
-   * @param string $type
-   *   Type of components: 'module' | 'theme' | 'profile'
-   * @param array $list
-   *   Array of theme or module names.
+   * @param \Drupal\Core\Extension\Extension[] $list
+   *   An associative array of Extension objects, keyed by extension name.
    *
    * @return array
    *   Folders indexed by configuration name.
    */
-  public function getComponentNames($type, array $list) {
+  public function getComponentNames(array $list) {
     $extension = '.' . $this->getFileExtension();
     $folders = array();
-    foreach ($list as $name) {
-      $directory = $this->getComponentFolder($type, $name);
+    foreach ($list as $extension_object) {
+      $directory = $this->getComponentFolder($extension_object);
       if (file_exists($directory)) {
         $files = new \GlobIterator(DRUPAL_ROOT . '/' . $directory . '/*' . $extension);
         foreach ($files as $file) {
@@ -196,18 +195,45 @@ public function getComponentNames($type, array $list) {
   }
 
   /**
+   * Get all configuration names and folders for Drupal core.
+   *
+   * @return array
+   *   Folders indexed by configuration name.
+   */
+  public function getCoreNames() {
+    $extension = '.' . $this->getFileExtension();
+    $folders = array();
+    $directory = $this->getCoreFolder();
+    if (file_exists($directory)) {
+      $files = new \GlobIterator(DRUPAL_ROOT . '/' . $directory . '/*' . $extension);
+      foreach ($files as $file) {
+        $folders[$file->getBasename($extension)] = $directory;
+      }
+    }
+    return $folders;
+  }
+
+  /**
    * Get folder inside each component that contains the files.
    *
-   * @param string $type
-   *   Component type: 'module' | 'theme' | 'profile'
-   * @param string $name
-   *   Component name.
+   * @param \Drupal\Core\Extension\Extension $extension
+   *   The Extension object for the component.
    *
    * @return string
    *   The configuration folder name for this component.
    */
-  protected function getComponentFolder($type, $name) {
-    return drupal_get_path($type, $name) . '/' . $this->getCollectionDirectory();
+  protected function getComponentFolder(Extension $extension) {
+    return $extension->getPath() . '/' . $this->getCollectionDirectory();
+  }
+
+  /**
+   * Get folder inside Drupal core that contains the files.
+   *
+   * @return string
+   *   The configuration folder name for core.
+   */
+  protected function getCoreFolder() {
+    return drupal_get_path('core', 'core') . '/' . $this->getCollectionDirectory();
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Extension/ModuleInstaller.php b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
index caddbad..af53b82 100644
--- a/core/lib/Drupal/Core/Extension/ModuleInstaller.php
+++ b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
@@ -18,7 +18,7 @@
 /**
  * Default implementation of the module installer.
  *
- * It registers the module in config, install its own configuration,
+ * It registers the module in config, installs its own configuration,
  * installs the schema, updates the Drupal kernel and more.
  */
 class ModuleInstaller implements ModuleInstallerInterface {
diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index 2c2a6a5..1520c5e 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -82,7 +82,7 @@ function _ckeditor_theme_css($theme = NULL) {
   if (!isset($theme)) {
     $theme = \Drupal::config('system.theme')->get('default');
   }
-  if ($theme_path = drupal_get_path('theme', $theme)) {
+  if (isset($theme) && $theme_path = drupal_get_path('theme', $theme)) {
     $info = system_get_info('theme', $theme);
     if (isset($info['ckeditor_stylesheets'])) {
       $css = $info['ckeditor_stylesheets'];
diff --git a/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php b/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php
index 3cb8ab0..0285432 100644
--- a/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php
+++ b/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php
@@ -27,7 +27,6 @@ class CommentStringIdEntitiesTest extends KernelTestBase {
     'user',
     'field',
     'field_ui',
-    'entity',
     'entity_test',
     'text',
   );
diff --git a/core/modules/config/tests/config_test/src/TestInstallStorage.php b/core/modules/config/tests/config_test/src/TestInstallStorage.php
index e4b3864..c4ee57e 100644
--- a/core/modules/config/tests/config_test/src/TestInstallStorage.php
+++ b/core/modules/config/tests/config_test/src/TestInstallStorage.php
@@ -23,13 +23,12 @@ class TestInstallStorage extends InstallStorage {
    */
   protected function getAllFolders() {
     if (!isset($this->folders)) {
-      $this->folders = $this->getComponentNames('core', array('core'));
-      // @todo Refactor getComponentNames() to use the extension list directly.
+      $this->folders = $this->getCoreNames();
       $listing = new ExtensionDiscovery(\Drupal::root());
       $listing->setProfileDirectories(array());
-      $this->folders += $this->getComponentNames('profile', array_keys($listing->scan('profile')));
-      $this->folders += $this->getComponentNames('module', array_keys($listing->scan('module')));
-      $this->folders += $this->getComponentNames('theme', array_keys($listing->scan('theme')));
+      $this->folders += $this->getComponentNames($listing->scan('profile'));
+      $this->folders += $this->getComponentNames($listing->scan('module'));
+      $this->folders += $this->getComponentNames($listing->scan('theme'));
     }
     return $this->folders;
   }
diff --git a/core/modules/quickedit/quickedit.module b/core/modules/quickedit/quickedit.module
index 6813fe4..38805a2 100644
--- a/core/modules/quickedit/quickedit.module
+++ b/core/modules/quickedit/quickedit.module
@@ -77,7 +77,7 @@ function quickedit_library_info_alter(&$libraries, $extension) {
 
     // First let the base theme modify the library, then the actual theme.
     $alter_library = function(&$library, $theme) use (&$alter_library) {
-      if ($theme_path = drupal_get_path('theme', $theme)) {
+      if (isset($theme) && $theme_path = drupal_get_path('theme', $theme)) {
         $info = system_get_info('theme', $theme);
         // Recurse to process base theme(s) first.
         if (isset($info['base theme'])) {
diff --git a/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php b/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
index f867045..eeea4aa 100644
--- a/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
+++ b/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
@@ -70,7 +70,20 @@ function testDrupalGetFilename() {
     // a fixed location and naming.
     $this->assertIdentical(drupal_get_filename('profile', 'standard'), 'core/profiles/standard/standard.info.yml');
 
+    // Generate a non-existing module name.
+    $non_existing_module = uniqid("", TRUE);
+
+    // Set a custom error handler so we can ignore the file not found error.
+    set_error_handler(function($severity, $message, $file, $line) {
+      // Skip error handling if this is a "file not found" error.
+      if (!(error_reporting() & $severity) || strstr($message, 'is missing from the file system:')) {
+        return;
+      }
+      throw new ErrorException($message, 0, $severity, $file, $line);
+    });
     // Searching for an item that does not exist returns NULL.
-    $this->assertNull(drupal_get_filename('module', uniqid("", TRUE)), 'Searching for an item that does not exist returns NULL.');
+    $this->assertNull(drupal_get_filename('module', $non_existing_module), 'Searching for an item that does not exist returns NULL.');
+    // Restore the original error handler.
+    restore_error_handler();
   }
 }
diff --git a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
index 3251305..8c9c217 100644
--- a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
+++ b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
@@ -74,7 +74,7 @@ function testDefault() {
    * Tests non-existing libraries.
    */
   function testLibraryUnknown() {
-    $build['#attached']['library'][] = 'unknown/unknown';
+    $build['#attached']['library'][] = 'core/unknown';
     $assets = AttachedAssets::createFromRenderArray($build);
 
     $this->assertIdentical([], $this->assetResolver->getJsAssets($assets, FALSE)[0], 'Unknown library was not added to the page.');
diff --git a/core/modules/system/src/Tests/Entity/EntitySchemaTest.php b/core/modules/system/src/Tests/Entity/EntitySchemaTest.php
index 6f6f39d..5ce61be 100644
--- a/core/modules/system/src/Tests/Entity/EntitySchemaTest.php
+++ b/core/modules/system/src/Tests/Entity/EntitySchemaTest.php
@@ -24,13 +24,6 @@ class EntitySchemaTest extends EntityUnitTestBase  {
   protected $database;
 
   /**
-   * Modules to enable.
-   *
-   * @var array
-   */
-  public static $modules = array('menu_link');
-
-  /**
    * {@inheritdoc}
    */
   public function setUp() {
diff --git a/core/modules/taxonomy/src/Tests/TermKernelTest.php b/core/modules/taxonomy/src/Tests/TermKernelTest.php
index 6692775..3d08826 100644
--- a/core/modules/taxonomy/src/Tests/TermKernelTest.php
+++ b/core/modules/taxonomy/src/Tests/TermKernelTest.php
@@ -23,7 +23,7 @@ class TermKernelTest extends KernelTestBase {
   /**
    * {@inheritdoc}
    */
-  public static $modules = array( 'filter', 'taxonomy', 'taxonomy_term', 'text', 'user' );
+  public static $modules = array( 'filter', 'taxonomy', 'text', 'user' );
 
   /**
    * {@inheritdoc}
diff --git a/core/modules/views/src/Tests/Plugin/RowEntityTest.php b/core/modules/views/src/Tests/Plugin/RowEntityTest.php
index 35bd1e2..7de0305 100644
--- a/core/modules/views/src/Tests/Plugin/RowEntityTest.php
+++ b/core/modules/views/src/Tests/Plugin/RowEntityTest.php
@@ -24,7 +24,7 @@ class RowEntityTest extends ViewUnitTestBase {
    *
    * @var array
    */
-  public static $modules = ['taxonomy', 'text', 'filter', 'field', 'entity', 'system', 'node', 'user'];
+  public static $modules = ['taxonomy', 'text', 'filter', 'field', 'system', 'node', 'user'];
 
   /**
    * Views used by this test.