diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 01b1393bba..135942637c 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -432,8 +432,8 @@ function install_begin_request($class_loader, &$install_state) {
   }
 
   // Before having installed the system module and being able to do a module
-  // rebuild, prime the drupal_get_filename() static cache with the module's
-  // exact location.
+  // rebuild, prime the drupal_get_filename() static cache with the system
+  // module's location.
   // @todo Remove as part of https://www.drupal.org/node/2186491
   drupal_get_filename('module', 'system', 'core/modules/system/system.info.yml');
 
@@ -1063,7 +1063,7 @@ function install_base_system(&$install_state) {
   file_ensure_htaccess();
 
   // Prime the drupal_get_filename() static cache with the user module's
-  // exact location.
+  // location.
   // @todo Remove as part of https://www.drupal.org/node/2186491
   drupal_get_filename('module', 'user', 'core/modules/user/user.info.yml');
 
diff --git a/core/lib/Drupal/Core/Extension/ExtensionList.php b/core/lib/Drupal/Core/Extension/ExtensionList.php
index 1c46cec8bd..5e3bc921eb 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionList.php
@@ -87,6 +87,8 @@
    * over the discovered extension folders.
    *
    * @var string[]
+   *
+   * @internal
    */
   protected $addedFileNames = [];
 
@@ -94,6 +96,8 @@
    * Static version of the added file names during the installer.
    *
    * @var string
+   *
+   * @internal
    */
   protected static $staticAddedFileNames;
 
@@ -159,15 +163,15 @@ public function reset() {
     $this->fileNames = NULL;
 
     try {
-      $this->state->delete($this->getFilenameCacheId());
+      $this->state->delete($this->getFilenamesCacheId());
     }
     catch (\Exception $e) {
       // Ignore exceptions caused by a non existing {key_value} table in the
       // early installer.
     }
 
-    $this->cache->delete($this->getFilenameCacheId());
-    // We explicitly don't reset the addedFileNames as its sort of a static
+    $this->cache->delete($this->getFilenamesCacheId());
+    // We explicitly don't reset the addedFileNames as it is sort of a static
     // cache.
     // @todo In the long run it would be great to add the reset, but the early
     //   installer fails due to that.
@@ -200,7 +204,7 @@ protected function getInfoCacheId() {
    * @return string
    *   The filename cache ID.
    */
-  protected function getFilenameCacheId() {
+  protected function getFilenamesCacheId() {
     return "system.{$this->type}.files";
   }
 
@@ -208,10 +212,11 @@ protected function getFilenameCacheId() {
    * Determines if an extension exists in the filesystem.
    *
    * @param string $extension_name
-   *   The machine name of the extension.
+   *   The extension machine name of the extension.
    *
    * @return bool
-   *   TRUE if the extension exists (whether installed or not) and FALSE if not.
+   *   TRUE if the extension exists (regardless installed or not) and FALSE if
+   *   not.
    */
   public function extensionExists($extension_name) {
     $extensions = $this->listExtensions();
@@ -222,13 +227,13 @@ public function extensionExists($extension_name) {
    * Returns the human-readable name of the extension.
    *
    * @param string $extension_name
-   *   The extension name.
+   *   The extension machine name.
    *
    * @return string
    *   The human-readable name of the extension.
    *
    * @throws \InvalidArgumentException
-   *   If there is no extension with the supplied machine name.
+   *   If there is no extension with the supplied extension human name.
    */
   public function getName($extension_name) {
     return $this->getExtension($extension_name)->info['name'];
@@ -238,10 +243,10 @@ public function getName($extension_name) {
    * Returns a single extension.
    *
    * @param string $extension_name
-   *   The extension name.
+   *   The extension machine name.
    *
    * @return \Drupal\Core\Extension\Extension
-   *   Processed extension object for the given machine name.
+   *   Processed extension object for the given extension machine name.
    *
    * @throws \InvalidArgumentException
    *   If there is no extension with the supplied name.
@@ -259,7 +264,7 @@ public function getExtension($extension_name) {
    * Returns all available extensions.
    *
    * @return \Drupal\Core\Extension\Extension[]
-   *   Processed extension objects, keyed by machine name.
+   *   Processed extension objects, keyed by extension machine name.
    */
   public function listExtensions() {
     if ($this->extensions !== NULL) {
@@ -282,17 +287,17 @@ public function listExtensions() {
    * extensions to this raw listing.
    *
    * @return \Drupal\Core\Extension\Extension[]
-   *   Unprocessed extension objects, keyed by machine name.
+   *   Unprocessed extension objects, keyed by extension machine name.
    */
   protected function doScanExtensions() {
     return $this->getExtensionDiscovery()->scan($this->type);
   }
 
   /**
-   * Build the actual list of extensions before caching it.
+   * Builds the list of extensions.
    *
    * @return \Drupal\Core\Extension\Extension[]
-   *   Processed extension objects, keyed by machine name.
+   *   Processed extension objects, keyed by extension machine name.
    *
    * @throws \Drupal\Core\Extension\InfoParserException
    *   If one of the .info.yml files is incomplete, or causes a parsing error.
@@ -310,7 +315,7 @@ protected function doListExtensions() {
       // contributed extensions to use for ordering extension lists.
       $extension->info['mtime'] = $extension->getMTime();
 
-      // Merge in defaults and save.
+      // Merge extension type-specific defaults.
       $extension->info += $this->defaults;
 
       // Invoke hook_system_info_alter() to give installed modules a chance to
@@ -356,13 +361,13 @@ public function getExtensionInfo($extension_name) {
    */
   public function getAllAvailableInfo() {
     if ($this->extensionInfo === NULL) {
-      $cache_key = "system.{$this->type}.info";
-      if ($cache = $this->cache->get($cache_key)) {
+      $cache_id = $this->getInfoCacheId();
+      if ($cache = $this->cache->get($cache_id)) {
         $info = $cache->data;
       }
       else {
         $info = $this->recalculateInfo();
-        $this->cache->set($cache_key, $info);
+        $this->cache->set($cache_id, $info);
       }
       $this->extensionInfo = $info;
     }
@@ -371,10 +376,10 @@ public function getAllAvailableInfo() {
   }
 
   /**
-   * Returns installed extension names.
+   * Returns installed extension machine names.
    *
    * @return string[]
-   *   The machine names of all installed extensions of this type.
+   *   The extension machine names of all installed extensions of this type.
    */
   abstract protected function getInstalledExtensionsNames();
 
@@ -395,11 +400,8 @@ public function getAllInstalledInfo() {
   /**
    * Generates the information from .info.yml files for extensions of this type.
    *
-   * The information is placed in cache with the "system.{extension_type}.info"
-   * key.
-   *
    * @return array[]
-   *   An array of arrays of .info.yml entries keyed by the extension name.
+   *   An array of arrays of .info.yml entries keyed by the extension machine name.
    */
   protected function recalculateInfo() {
     return array_map(function (Extension $extension) {
@@ -408,20 +410,18 @@ protected function recalculateInfo() {
   }
 
   /**
-   * Returns a list of extension file name keyed by extension name.
+   * Returns a list of extension file name keyed by extension machine name.
    *
    * @return string[]
    */
   public function getFilenames() {
     if ($this->fileNames === NULL) {
-      $cache_id = $this->getFilenameCacheId();
+      $cache_id = $this->getFilenamesCacheId();
       if ($cache = $this->cache->get($cache_id)) {
         $file_names = $cache->data;
       }
       // We use $file_names below.
-      elseif ($file_names = $this->state->get($cache_id)) {
-      }
-      else {
+      elseif (!$file_names = $this->state->get($cache_id)) {
         $file_names = $this->recalculateFilenames();
         // Store filenames to allow static::getFilename() to retrieve them
         // without having to rebuild or scan the filesystem.
@@ -436,11 +436,8 @@ public function getFilenames() {
   /**
    * Generates a sorted list of .info.yml file locations for all extensions.
    *
-   * The information is placed in cache with the "system.{extension_type}.files"
-   * key.
-   *
    * @return string[]
-   *   An array of .info.yml file locations keyed by the extension name.
+   *   An array of .info.yml file locations keyed by the extension machine name.
    */
   protected function recalculateFilenames() {
     $extensions = $this->listExtensions();
@@ -483,7 +480,7 @@ public function setFilename($extension_name, $filename) {
   }
 
   /**
-   * Gets the filename for a system resource.
+   * Gets the filename for an extension.
    *
    * The filename, whether provided, cached, or retrieved from the database, is
    * only returned if the file exists.
@@ -493,17 +490,22 @@ public function setFilename($extension_name, $filename) {
    * depending on a site's configuration. For example, a module 'foo' may
    * legally be located in any of these four places:
    *
-   * core/modules/foo/foo.info.yml
-   * modules/foo/foo.info.yml
-   * sites/all/modules/foo/foo.info.yml
-   * sites/example.com/modules/foo/foo.info.yml
+   * - core/modules/foo/foo.info.yml
+   * - modules/foo/foo.info.yml
+   * - sites/all/modules/foo/foo.info.yml
+   * - sites/example.com/modules/foo/foo.info.yml
    *
    * while a theme 'bar' may be located in any of the following four places:
    *
-   * core/themes/bar/bar.info.yml
-   * themes/bar/bar.info.yml
-   * sites/all/themes/bar/bar.info.yml
-   * sites/example.com/themes/bar/bar.info.yml
+   * - core/themes/bar/bar.info.yml
+   * - themes/bar/bar.info.yml
+   * - sites/all/themes/bar/bar.info.yml
+   * - sites/example.com/themes/bar/bar.info.yml
+   *
+   * An installation profile maybe be located in any of the following places:
+   *
+   * - core/profiles/baz/baz.info.yml
+   * - /profiles/baz/baz.info.yml
    *
    * Calling ExtensionList::getFilename('foo') will give you one of the above,
    * depending on where the extension is located and what type it is.
@@ -541,7 +543,7 @@ public function getFilename($extension_name) {
    *   The name of the extension for which the path is requested.
    *
    * @return string
-   *   The drupal-root-relative path to the specified extension.
+   *   The Drupal-root-relative path to the specified extension.
    *
    * @throws \InvalidArgumentException
    *   If there is no extension with the supplied name.
diff --git a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
index 5e8be979da..3bcac67105 100644
--- a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
@@ -40,10 +40,6 @@ class ModuleExtensionList extends ExtensionList {
   protected $profileList;
 
   /**
-   */
-  protected $moduleLocations;
-
-  /**
    * Constructs a new ModuleExtensionList instance.
    *
    * @param string $root
@@ -73,7 +69,7 @@ public function __construct($root, $type, CacheBackendInterface $cache, InfoPars
     $this->configFactory = $config_factory;
     $this->profileList = $profile_list;
 
-    // Use the information from the container.
+    // Use the information from the container. This is an optimization.
     foreach ($container_modules_info as $module_name => $info) {
       $this->setFilename($module_name, $info['pathname']);
     }
@@ -98,7 +94,7 @@ protected function getExtensionDiscovery() {
    * @param \Drupal\Core\Extension\ExtensionDiscovery $discovery
    *   The extension discovery.
    *
-   * @return array
+   * @return string[]
    *   Paths to all installation profiles.
    */
   protected function getProfileDirectories(ExtensionDiscovery $discovery) {
@@ -160,7 +156,6 @@ protected function doScanExtensions() {
    * {@inheritdoc}
    */
   protected function doListExtensions() {
-
     // Find modules.
     $extensions = parent::doListExtensions();
     // It is possible that a module was marked as required by
@@ -190,7 +185,7 @@ protected function doListExtensions() {
         $active_profile->info['hidden'] = TRUE;
       }
 
-      // The installation profile is required, if it's a valid module.
+      // The installation profile is required.
       $active_profile->info['required'] = TRUE;
       // Add a default distribution name if the profile did not provide one.
       // @see install_profile_info()
@@ -224,7 +219,7 @@ protected function ensureRequiredDependencies(Extension $module, array $modules
         $dependency_name = ModuleHandler::parseDependency($dependency)['name'];
         if (!isset($modules[$dependency_name]->info['required'])) {
           $modules[$dependency_name]->info['required'] = TRUE;
-          $modules[$dependency_name]->info['explanation'] = $this->t('Dependency of required module @module', array('@module' => $module->info['name']));
+          $modules[$dependency_name]->info['explanation'] = $this->t('Dependency of required module @module', ['@module' => $module->info['name']]);
           // Ensure any dependencies it has are required.
           $this->ensureRequiredDependencies($modules[$dependency_name], $modules);
         }