diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 5d442ac..4800900 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -701,14 +701,13 @@ function drupal_get_filename($type, $name, $filename = NULL) {
     $files[$type][$name] = $filename;
   }
   elseif (!isset($files[$type][$name])) {
-    $container = \Drupal::getContainer();
-    // For modules, first consult ModuleHandler for the current module list.
-    if ($type == 'module' && !isset($files[$type]) && $container->has('module_handler')) {
+    // For modules, consult ModuleHandler for the current module list.
+    if ($type == 'module' && !isset($files[$type]) && \Drupal::hasService('module_handler')) {
       $files[$type] = \Drupal::moduleHandler()->getModuleList();
     }
-    // Consult the extension lists prepared by system_rebuild_module_data() and
+    // Consult the extension list prepared by system_rebuild_module_data() and
     // system_rebuild_theme_data() in State.
-    if (!isset($files[$type][$name]) && $container->has('state')) {
+    if (!isset($files[$type][$name]) && \Drupal::hasService('state')) {
       $file_list = \Drupal::state()->get('system.' . $type . '.files');
       if ($file_list && isset($file_list[$name]) && file_exists(DRUPAL_ROOT . '/' . $file_list[$name])) {
         $files[$type][$name] = $file_list[$name];
@@ -716,13 +715,12 @@ function drupal_get_filename($type, $name, $filename = NULL) {
     }
     // Fallback to ExtensionDiscovery.
     if (!isset($files[$type][$name])) {
-      $dir = ($type == 'theme_engine' ? 'themes/engines' : $original_type . 's');
       $listing = new ExtensionDiscovery();
       // Prevent an infinite recursion by this legacy function.
       if ($original_type == 'profile') {
         $listing->setProfileDirectories(array());
       }
-      foreach ($listing->scan($dir) as $extension_name => $file) {
+      foreach ($listing->scan($original_type) as $extension_name => $file) {
         $files[$type][$extension_name] = $file->uri;
       }
     }
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index fd1bc6f..7b06081 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -503,7 +503,7 @@ function install_begin_request(&$install_state) {
   // Add list of all available profiles to the installation state.
   $listing = new ExtensionDiscovery();
   $listing->setProfileDirectories(array());
-  $install_state['profiles'] += $listing->scan('profiles');
+  $install_state['profiles'] += $listing->scan('profile');
 
   // Prime drupal_get_filename()'s static cache.
   foreach ($install_state['profiles'] as $name => $profile) {
diff --git a/core/includes/install.inc b/core/includes/install.inc
index af5f773..c268eb7 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -568,7 +568,7 @@ function drupal_verify_profile($install_state) {
   // Get the list of available modules for the selected installation profile.
   $listing = new ExtensionDiscovery();
   $present_modules = array();
-  foreach ($listing->scan('modules') as $present_module) {
+  foreach ($listing->scan('module') as $present_module) {
     $present_modules[] = $present_module->name;
   }
 
diff --git a/core/includes/module.inc b/core/includes/module.inc
index 66fa1be..453de05 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -287,7 +287,7 @@ function module_uninstall($module_list = array(), $uninstall_dependents = TRUE)
  */
 function drupal_required_modules() {
   $listing = new ExtensionDiscovery();
-  $files = $listing->scan('modules');
+  $files = $listing->scan('module');
   $required = array();
 
   // Unless called by the installer, an installation profile is required and
diff --git a/core/lib/Drupal/Core/Config/InstallStorage.php b/core/lib/Drupal/Core/Config/InstallStorage.php
index 5d05790..f86ec10 100644
--- a/core/lib/Drupal/Core/Config/InstallStorage.php
+++ b/core/lib/Drupal/Core/Config/InstallStorage.php
@@ -117,8 +117,8 @@ protected function getAllFolders() {
         $this->folders += $this->getComponentNames('profile', array($profile));
       }
       $listing = new ExtensionDiscovery();
-      $this->folders += $this->getComponentNames('module', array_keys($listing->scan('modules')));
-      $this->folders += $this->getComponentNames('theme', array_keys($listing->scan('themes')));
+      $this->folders += $this->getComponentNames('module', array_keys($listing->scan('module')));
+      $this->folders += $this->getComponentNames('theme', array_keys($listing->scan('theme')));
     }
     return $this->folders;
   }
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 73c4276..2183096 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -300,7 +300,7 @@ protected function moduleData($module) {
       // First, find profiles.
       $listing = new ExtensionDiscovery();
       $listing->setProfileDirectories(array());
-      $all_profiles = $listing->scan('profiles');
+      $all_profiles = $listing->scan('profile');
       $profiles = array_intersect_key($all_profiles, $this->moduleList);
 
       // If a module is within a profile directory but specifies another
@@ -319,7 +319,7 @@ protected function moduleData($module) {
       $listing->setProfileDirectories($profile_directories);
 
       // Now find modules.
-      $this->moduleData = $profiles + $listing->scan('modules');
+      $this->moduleData = $profiles + $listing->scan('module');
     }
     return isset($this->moduleData[$module]) ? $this->moduleData[$module] : FALSE;
   }
diff --git a/core/lib/Drupal/Core/Extension/Extension.php b/core/lib/Drupal/Core/Extension/Extension.php
index c810121..173b779 100644
--- a/core/lib/Drupal/Core/Extension/Extension.php
+++ b/core/lib/Drupal/Core/Extension/Extension.php
@@ -61,7 +61,7 @@ class Extension implements \Serializable {
   protected $splFileInfo;
 
   /*
-  // @see __get()
+  // @todo See __get() below.
   protected $infoParser;
   protected $info;
   */
@@ -188,7 +188,9 @@ public function unserialize($data) {
       $this->$property = $value;
     }
     /*
-    // @see serialize()
+    // @todo Not possible as long as ThemeHandler::listInfo(),
+    //   ThemeHandler::rebuildThemeData(), and system_list() are adding
+    //   arbitrary custom properties to the Extension object.
     $this->type = $data['type'];
     $this->infoPathname = $data['pathname'];
     $this->name = basename($data['pathname'], '.info.yml');
diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index ffb2745..c0ac5e9 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -77,7 +77,7 @@ class ExtensionDiscovery {
    * modules:
    * @code
    * $listing = new ExtensionDiscovery();
-   * $modules = $listing->scan('modules');
+   * $modules = $listing->scan('module');
    * @endcode
    *
    * The following directories will be searched (in the order stated):
@@ -92,10 +92,9 @@ class ExtensionDiscovery {
    * will take precedence over extensions found earlier - unless they are not
    * compatible with the current version of Drupal core.
    *
-   * @param string $directory
-   *   The subdirectory name in which to search for extensions. For example,
-   *   'modules' will recursively search all 'modules' directories for
-   *   extensions of type 'module'.
+   * @param string $type
+   *   The extension type to search for. One of 'profile', 'module', 'theme', or
+   *   'theme_engine'.
    * @param bool $include_tests
    *   (optional) Whether to include test extensions. Defaults to FALSE when not
    *   in a test environment; i.e., all 'tests' directories are excluded in the
@@ -104,16 +103,7 @@ class ExtensionDiscovery {
    * @return \Drupal\Core\Extension\Extension[]
    *   An associative array of Extension objects, keyed by extension name.
    */
-  public function scan($directory, $include_tests = NULL) {
-    // @todo Fix callers to pass $type.
-    // @see drupal_get_filename()
-    if ($directory == 'themes/engines') {
-      $type = 'theme_engine';
-    }
-    else {
-      $type = substr($directory, 0, -1);
-    }
-
+  public function scan($type, $include_tests = NULL) {
     // Determine the installation profile directories to scan for extensions,
     // unless explicit profile directories have been set.
     if (!isset($this->profileDirectories)) {
diff --git a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
index 024eeb6..3012627 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
@@ -71,7 +71,8 @@ public function setModuleList(array $module_list = array());
    *
    * @param array $modules
    *   An array of module objects keyed by module name. Each object contains
-   *   information discovered during a Drupal\Core\Extension\ExtensionDiscovery scan.
+   *   information discovered during a Drupal\Core\Extension\ExtensionDiscovery
+   *   scan.
    *
    * @return
    *   The same array with the new keys for each module:
diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index 696f432..d7eef35 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -243,8 +243,8 @@ public function reset() {
    */
   public function rebuildThemeData() {
     $listing = $this->getExtensionDiscovery();
-    $themes = $listing->scan('themes');
-    $engines = $listing->scan('themes/engines');
+    $themes = $listing->scan('theme');
+    $engines = $listing->scan('theme_engine');
 
     // Set defaults for theme info.
     $defaults = array(
diff --git a/core/modules/config/tests/config_test/lib/Drupal/config_test/TestInstallStorage.php b/core/modules/config/tests/config_test/lib/Drupal/config_test/TestInstallStorage.php
index 4782304..bae2dd0 100644
--- a/core/modules/config/tests/config_test/lib/Drupal/config_test/TestInstallStorage.php
+++ b/core/modules/config/tests/config_test/lib/Drupal/config_test/TestInstallStorage.php
@@ -24,9 +24,9 @@ class TestInstallStorage extends InstallStorage {
   protected function getAllFolders() {
     if (!isset($this->folders)) {
       $listing = new ExtensionDiscovery();
-      $this->folders = $this->getComponentNames('profile', array_keys($listing->scan('profiles')));
-      $this->folders += $this->getComponentNames('module', array_keys($listing->scan('modules')));
-      $this->folders += $this->getComponentNames('theme', array_keys($listing->scan('themes')));
+      $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')));
     }
     return $this->folders;
   }
diff --git a/core/modules/config/tests/config_test/lib/Drupal/config_test/TestSchemaStorage.php b/core/modules/config/tests/config_test/lib/Drupal/config_test/TestSchemaStorage.php
index bf5a6fc..aae81c3 100644
--- a/core/modules/config/tests/config_test/lib/Drupal/config_test/TestSchemaStorage.php
+++ b/core/modules/config/tests/config_test/lib/Drupal/config_test/TestSchemaStorage.php
@@ -32,9 +32,9 @@ protected function getAllFolders() {
     if (!isset($this->folders)) {
       $listing = new ExtensionDiscovery();
       $this->folders = $this->getBaseDataTypeSchema();
-      $this->folders += $this->getComponentNames('profile', array_keys($listing->scan('profiles')));
-      $this->folders += $this->getComponentNames('module', array_keys($listing->scan('modules')));
-      $this->folders += $this->getComponentNames('theme', array_keys($listing->scan('themes')));
+      $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')));
     }
     return $this->folders;
   }
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 58289e7..12e9814 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -462,7 +462,7 @@ function simpletest_test_get_all($module = NULL) {
     else {
       // Select all PSR-0 classes in the Tests namespace of all modules.
       $listing = new ExtensionDiscovery();
-      $all_data = $listing->scan('modules', TRUE);
+      $all_data = $listing->scan('module', TRUE);
       // If module is set then we keep only that one module.
       if (isset($module)) {
         $all_data = array(
@@ -470,8 +470,8 @@ function simpletest_test_get_all($module = NULL) {
         );
       }
       else {
-        $all_data += $listing->scan('profiles', TRUE);
-        $all_data += $listing->scan('themes', TRUE);
+        $all_data += $listing->scan('profile', TRUE);
+        $all_data += $listing->scan('theme', TRUE);
       }
       $classes = array();
       foreach ($all_data as $name => $data) {
@@ -551,12 +551,11 @@ function simpletest_test_get_all($module = NULL) {
 function simpletest_classloader_register() {
   // Use the same cache prefix as simpletest_test_get_all().
   $cid = "simpletest::all";
-  // @see drupal_get_filename()
   $types = array(
-    'theme_engine' => 'themes/engines',
-    'module' => 'modules',
-    'theme' => 'themes',
-    'profile' => 'profiles',
+    'theme_engine',
+    'module',
+    'theme',
+    'profile',
   );
 
   if ($cache = cache()->get($cid)) {
@@ -565,8 +564,8 @@ function simpletest_classloader_register() {
   else {
     $listing = new ExtensionDiscovery();
     $extensions = array();
-    foreach ($types as $type => $dir) {
-      foreach ($listing->scan($dir, TRUE) as $name => $file) {
+    foreach ($types as $type) {
+      foreach ($listing->scan($type, TRUE) as $name => $file) {
         $extensions[$type][$name] = $file->uri;
       }
     }
@@ -574,7 +573,7 @@ function simpletest_classloader_register() {
   }
 
   $classloader = drupal_classloader();
-  foreach ($types as $type => $info) {
+  foreach ($types as $type) {
     foreach ($extensions[$type] as $name => $uri) {
       drupal_classloader_register($name, dirname($uri));
       $classloader->add('Drupal\\' . $name . '\\Tests', DRUPAL_ROOT . '/' . dirname($uri) . '/tests');
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/SystemListingTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/SystemListingTest.php
index 742d8da..d37fd4b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/SystemListingTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/SystemListingTest.php
@@ -58,7 +58,7 @@ function testDirectoryPrecedence() {
     // expected.
     $listing = new ExtensionDiscovery();
     $listing->setProfileDirectories(array('core/profiles/testing'));
-    $files = $listing->scan('modules');
+    $files = $listing->scan('module');
     foreach ($expected_directories as $module => $directories) {
       $expected_directory = array_shift($directories);
       $expected_uri = "$expected_directory/$module/$module.module";
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php
index ea93407..dda6fb0 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php
@@ -84,7 +84,7 @@ function setUp() {
 
     // Enable all available non-testing themes.
     $listing = new ExtensionDiscovery();
-    $this->themes = $listing->scan('themes', FALSE);
+    $this->themes = $listing->scan('theme', FALSE);
     theme_enable(array_keys($this->themes));
 
     // Create a test user.
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 1d36e5e..ab22256 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2562,10 +2562,10 @@ function system_get_info($type, $name = NULL) {
 function _system_rebuild_module_data() {
   $listing = new ExtensionDiscovery();
   // Find modules
-  $modules = $listing->scan('modules');
+  $modules = $listing->scan('module');
 
   // Find installation profiles.
-  $profiles = $listing->scan('profiles');
+  $profiles = $listing->scan('profile');
 
   // Include the installation profile in modules that are loaded.
   if ($profile = drupal_get_profile()) {