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..00c06d6 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -302,6 +302,11 @@ function install_begin_request($class_loader, &$install_state) {
   // Allow command line scripts to override server variables used by Drupal.
   require_once __DIR__ . '/bootstrap.inc';
 
+  // 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.
+  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
@@ -1029,6 +1034,9 @@ function install_base_system(&$install_state) {
   // system.module in order to work.
   file_ensure_htaccess();
 
+  // Before installing the user module, perform a module rebuild so we know
+  // where the user module is located.
+  system_rebuild_module_data();
   // Enable the user module so that sessions can be recorded during the
   // upcoming bootstrap step.
   \Drupal::service('module_installer')->install(array('user'), FALSE);
diff --git a/core/includes/install.inc b/core/includes/install.inc
index 7e0e32e..8d9ef23 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -556,7 +556,7 @@ function drupal_verify_profile($install_state) {
   $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[] = $present_module->getName();
@@ -922,11 +922,19 @@ function drupal_check_profile($profile, array $install_state) {
 
   // Collect requirement testing results.
   $requirements = array();
+
+  // Performs an ExtensionDiscovery scan as the system module is unavailable and
+  // we don't yet know where all the modules are located.
+  $listing = new ExtensionDiscovery(\Drupal::root());
+  $module_list = $listing->scan('module');
   foreach ($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..70f9737 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 +81,46 @@ 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 $module) {
+          if (isset($module_list_scan[$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 $theme) {
+          if (isset($theme_list_scan[$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 = $listing->scan('profile');
+          if (isset($profile_list[$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..a8850da 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 +158,18 @@ 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();
+      // Perform an ExtensionDiscovery scan as we cannot use drupal_get_path()
+      // yet because the system module may not yet be enabled during install.
+      $listing = new ExtensionDiscovery(\Drupal::root());
       if ($profile = drupal_get_profile()) {
-        $this->folders += $this->getComponentNames('profile', array($profile));
+        $profile_list = $listing->scan('profile');
+        if (isset($profile_list[$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;
   }
@@ -172,21 +177,21 @@ 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) {
+      // 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)] = $directory;
         }
@@ -196,18 +201,45 @@ public function getComponentNames($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/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index fbd8f1b..bb4f201 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -134,8 +134,9 @@ public function __construct($root) {
    */
   public function scan($type, $include_tests = NULL) {
     // Determine the installation profile directories to scan for extensions,
-    // unless explicit profile directories have been set.
-    if (!isset($this->profileDirectories)) {
+    // unless explicit profile directories have been set. Exclude profiles
+    // as we can't have profiles within profiles.
+    if (!isset($this->profileDirectories) && $type != 'profile') {
       $this->setProfileDirectoriesFromSettings();
     }
 
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/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index 57a317e..0b31c40 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -466,7 +466,8 @@ public function rebuildThemeData() {
     );
 
     $sub_themes = array();
-    $files = array();
+    $files_theme = array();
+    $files_theme_engine = array();
     // Read info files for each theme.
     foreach ($themes as $key => $theme) {
       // @todo Remove all code that relies on the $status property.
@@ -495,6 +496,7 @@ public function rebuildThemeData() {
       if (isset($engines[$engine])) {
         $theme->owner = $engines[$engine]->getExtensionPathname();
         $theme->prefix = $engines[$engine]->getName();
+        $files_theme_engine[$engine] = $engines[$engine]->getPathname();
       }
 
       // Prefix screenshot with theme path.
@@ -502,7 +504,7 @@ public function rebuildThemeData() {
         $theme->info['screenshot'] = $theme->getPath() . '/' . $theme->info['screenshot'];
       }
 
-      $files[$key] = $theme->getPathname();
+      $files_theme[$key] = $theme->getPathname();
     }
     // Build dependencies.
     // @todo Move into a generic ExtensionHandler base class.
@@ -510,8 +512,10 @@ public function rebuildThemeData() {
     $themes = $this->moduleHandler->buildModuleDependencies($themes);
 
     // Store filenames to allow system_list() and drupal_get_filename() to
-    // retrieve them without having to scan the filesystem.
-    $this->state->set('system.theme.files', $files);
+    // retrieve them for themes and theme engines without having to scan the
+    // filesystem.
+    $this->state->set('system.theme.files', $files_theme);
+    $this->state->set('system.theme_engine.files', $files_theme_engine);
 
     // After establishing the full list of available themes, fill in data for
     // sub-themes.
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/src/Tests/ConfigDependencyTest.php b/core/modules/config/src/Tests/ConfigDependencyTest.php
index c78eaac..7bfb3aa 100644
--- a/core/modules/config/src/Tests/ConfigDependencyTest.php
+++ b/core/modules/config/src/Tests/ConfigDependencyTest.php
@@ -184,6 +184,9 @@ public function testConfigEntityUninstall() {
       )
     );
     $entity2->save();
+    // Perform a module rebuild so we can know where the node module is located.
+    // and uninstall it.
+    system_rebuild_module_data();
     // Test that doing a config uninstall of the node module deletes entity2
     // since it is dependent on entity1 which is dependent on the node module.
     $config_manager->uninstall('module', 'node');
diff --git a/core/modules/config/tests/config_test/src/TestInstallStorage.php b/core/modules/config/tests/config_test/src/TestInstallStorage.php
index e4b3864..c4ee57e 100644
--- a/core/modules/config/tests/config_test/src/TestInstallStorage.php
+++ b/core/modules/config/tests/config_test/src/TestInstallStorage.php
@@ -23,13 +23,12 @@ class TestInstallStorage extends InstallStorage {
    */
   protected function getAllFolders() {
     if (!isset($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/simpletest/src/KernelTestBase.php b/core/modules/simpletest/src/KernelTestBase.php
index bd611f4..48518a7 100644
--- a/core/modules/simpletest/src/KernelTestBase.php
+++ b/core/modules/simpletest/src/KernelTestBase.php
@@ -13,6 +13,7 @@
 use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Entity\Sql\SqlEntityStorageInterface;
+use Drupal\Core\Extension\ExtensionDiscovery;
 use Drupal\Core\KeyValueStore\KeyValueMemoryFactory;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Site\Settings;
@@ -483,6 +484,13 @@ protected function installEntitySchema($entity_type_id) {
    *   The new modules are only added to the active module list and loaded.
    */
   protected function enableModules(array $modules) {
+    // Perform an ExtensionDiscovery scan as this function may receive a
+    // profile that is not the current profile, and we don't yet have a cached
+    // way to receive inactive profile information.
+    $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');
     // Set the list of modules in the extension handler.
     $module_handler = $this->container->get('module_handler');
 
@@ -492,7 +500,7 @@ protected function enableModules(array $modules) {
     $extensions = $active_storage->read('core.extension');
 
     foreach ($modules as $module) {
-      $module_handler->addModule($module, drupal_get_path('module', $module));
+      $module_handler->addModule($module, $module_list[$module]->getPath());
       // Maintain the list of enabled modules in configuration.
       $extensions['module'][$module] = 0;
     }
diff --git a/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php b/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
index f867045..94802fd 100644
--- a/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
+++ b/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
@@ -17,34 +17,6 @@
 class GetFilenameUnitTest extends KernelTestBase {
 
   /**
-   * The container used by the test, moved out of the way.
-   *
-   * @var \Symfony\Component\DependencyInjection\ContainerInterface
-   */
-  protected $previousContainer;
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    parent::setUp();
-    // Store the previous container.
-    $this->previousContainer = $this->container;
-    $this->container = NULL;
-    \Drupal::unsetContainer();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function tearDown() {
-    parent::tearDown();
-    // Restore the previous container.
-    $this->container = $this->previousContainer;
-    \Drupal::setContainer($this->previousContainer);
-  }
-
-  /**
    * Tests that drupal_get_filename() works when the file is not in database.
    */
   function testDrupalGetFilename() {
@@ -53,24 +25,40 @@ function testDrupalGetFilename() {
     global $install_state;
     $install_state['parameters']['profile'] = 'testing';
 
-    // Assert that this test is meaningful.
-    $this->assertNull($this->container);
-    $this->assertFalse(\Drupal::hasContainer());
+    // Rebuild system.module.files state data.
+    drupal_static_reset('system_rebuild_module_data');
+    system_rebuild_module_data();
 
     // Retrieving the location of a module.
     $this->assertIdentical(drupal_get_filename('module', 'system'), 'core/modules/system/system.info.yml');
 
     // Retrieving the location of a theme.
+    \Drupal::service('theme_handler')->install(array('stark'));
     $this->assertIdentical(drupal_get_filename('theme', 'stark'), 'core/themes/stark/stark.info.yml');
 
     // Retrieving the location of a theme engine.
-    $this->assertIdentical(drupal_get_filename('theme_engine', 'phptemplate'), 'core/themes/engines/phptemplate/phptemplate.info.yml');
+    $this->assertIdentical(drupal_get_filename('theme_engine', 'twig'), 'core/themes/engines/twig/twig.info.yml');
 
     // Retrieving the location of a profile. Profiles are a special case with
     // a fixed location and naming.
-    $this->assertIdentical(drupal_get_filename('profile', 'standard'), 'core/profiles/standard/standard.info.yml');
+    $this->assertIdentical(drupal_get_filename('profile', 'testing'), 'core/profiles/testing/testing.info.yml');
+
+
+    // Generate a non-existing module name.
+    $non_existing_module = uniqid("", TRUE);
 
-    // Searching for an item that does not exist returns NULL.
-    $this->assertNull(drupal_get_filename('module', uniqid("", TRUE)), 'Searching for an item that does not exist returns NULL.');
+    // Set a custom error handler so we can ignore the file not found error.
+    set_error_handler(function($severity, $message, $file, $line) {
+      // Skip error handling if this is a "file not found" error.
+      if (strstr($message, 'is missing from the file system:')) {
+        \Drupal::state()->set('get_filename_test_triggered_error', TRUE);
+        return;
+      }
+      throw new \ErrorException($message, 0, $severity, $file, $line);
+    });
+    $this->assertNull(drupal_get_filename('module', $non_existing_module), 'Searching for an item that does not exist returns NULL.');
+    $this->assertTrue(\Drupal::state()->get('get_filename_test_triggered_error'), 'Searching for an item that does not exist triggers an error.');
+    // 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/system/src/Tests/Extension/ModuleHandlerTest.php b/core/modules/system/src/Tests/Extension/ModuleHandlerTest.php
index d2d6fed..c4eeddc 100644
--- a/core/modules/system/src/Tests/Extension/ModuleHandlerTest.php
+++ b/core/modules/system/src/Tests/Extension/ModuleHandlerTest.php
@@ -23,6 +23,13 @@ class ModuleHandlerTest extends KernelTestBase {
    */
   public static $modules = array('system');
 
+  public function setUp() {
+    parent::setUp();
+    // Set up the state values so we know where to find the files when running
+    // drupal_get_filename().
+    system_rebuild_module_data();
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -37,6 +44,10 @@ public function containerBuild(ContainerBuilder $container) {
    * The basic functionality of retrieving enabled modules.
    */
   function testModuleList() {
+    // Prime the drupal_get_filename() static cache with the location of the
+    // testing profile as it is not the currently active profile and we don't
+    // yet have any cached way to retrieve its location.
+    drupal_get_filename('profile', 'testing', 'core/profiles/testing/testing.info.yml');
     // Build a list of modules, sorted alphabetically.
     $profile_info = install_profile_info('testing', 'en');
     $module_list = $profile_info['dependencies'];
@@ -184,6 +195,10 @@ function testUninstallProfileDependency() {
     $profile = 'minimal';
     $dependency = 'dblog';
     $this->settingsSet('install_profile', $profile);
+    // Prime the drupal_get_filename() static cache with the location of the
+    // minimal profile as it is not the currently active profile and we don't
+    // yet have any cached way to retrieve its location.
+    drupal_get_filename('profile', 'minimal', 'core/profiles/' . $profile . '/' . $profile . '.info.yml');
     $this->enableModules(array('module_test', $profile));
 
     drupal_static_reset('system_rebuild_module_data');
diff --git a/core/modules/system/src/Tests/Extension/ThemeHandlerTest.php b/core/modules/system/src/Tests/Extension/ThemeHandlerTest.php
index 165c52f..d3dec11 100644
--- a/core/modules/system/src/Tests/Extension/ThemeHandlerTest.php
+++ b/core/modules/system/src/Tests/Extension/ThemeHandlerTest.php
@@ -316,6 +316,8 @@ function testThemeInfoAlter() {
     $themes = $this->themeHandler()->listInfo();
     $this->assertFalse(isset($themes[$name]->info['regions']['test_region']));
 
+    // Rebuild module data so we know where module_test is located.
+    system_rebuild_module_data();
     $this->moduleInstaller()->install(array('module_test'), FALSE);
     $this->assertTrue($this->moduleHandler()->moduleExists('module_test'));
 
diff --git a/core/modules/system/src/Tests/File/ScanDirectoryTest.php b/core/modules/system/src/Tests/File/ScanDirectoryTest.php
index 8e631d3..1eee62e 100644
--- a/core/modules/system/src/Tests/File/ScanDirectoryTest.php
+++ b/core/modules/system/src/Tests/File/ScanDirectoryTest.php
@@ -28,7 +28,10 @@ class ScanDirectoryTest extends FileTestBase {
 
   protected function setUp() {
     parent::setUp();
-    $this->path = drupal_get_path('module', 'simpletest') . '/files';
+    // Hardcode the location of the simpletest files as it is already known
+    // and shouldn't change, and we don't yet have a way to retreive their
+    // location from drupal_get_filename() in a cached way.
+    $this->path = 'core/modules/simpletest/files';
   }
 
   /**
@@ -123,10 +126,10 @@ function testOptionKey() {
    * Check that the recurse option descends into subdirectories.
    */
   function testOptionRecurse() {
-    $files = file_scan_directory(drupal_get_path('module', 'simpletest'), '/^javascript-/', array('recurse' => FALSE));
+    $files = file_scan_directory($this->path . '/..', '/^javascript-/', array('recurse' => FALSE));
     $this->assertTrue(empty($files), "Without recursion couldn't find javascript files.");
 
-    $files = file_scan_directory(drupal_get_path('module', 'simpletest'), '/^javascript-/', array('recurse' => TRUE));
+    $files = file_scan_directory($this->path . '/..', '/^javascript-/', array('recurse' => TRUE));
     $this->assertEqual(2, count($files), 'With recursion we found the expected javascript files.');
   }
 
diff --git a/core/modules/system/src/Tests/Installer/InstallerLanguageTest.php b/core/modules/system/src/Tests/Installer/InstallerLanguageTest.php
index a025b35..f7be5e6 100644
--- a/core/modules/system/src/Tests/Installer/InstallerLanguageTest.php
+++ b/core/modules/system/src/Tests/Installer/InstallerLanguageTest.php
@@ -30,7 +30,7 @@ function testInstallerTranslationFiles() {
       'it' => array(),
     );
 
-    $file_translation = new FileTranslation(drupal_get_path('module', 'simpletest') . '/files/translations');
+    $file_translation = new FileTranslation('core/modules/simpletest/files/translations');
     foreach ($expected_translation_files as $langcode => $files_expected) {
       $files_found = $file_translation->findTranslationFiles($langcode);
       $this->assertTrue(count($files_found) == count($files_expected), format_string('@count installer languages found.', array('@count' => count($files_expected))));
diff --git a/core/modules/system/src/Tests/Path/UrlAliasFixtures.php b/core/modules/system/src/Tests/Path/UrlAliasFixtures.php
index 777ffb1..1d0fd9b 100644
--- a/core/modules/system/src/Tests/Path/UrlAliasFixtures.php
+++ b/core/modules/system/src/Tests/Path/UrlAliasFixtures.php
@@ -80,6 +80,9 @@ public function sampleUrlAliases() {
   public function tableDefinition() {
     $tables = array();
 
+    // Prime the drupal_get_filename() cache with the location of the system
+    // module as its location is known and shouldn't change.
+    drupal_get_filename('module', 'system', 'core/modules/system/system.info.yml');
     module_load_install('system');
     $schema = system_schema();
 
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index f2dcc7c..204d0e3 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -883,14 +883,23 @@ function system_get_info($type, $name = NULL) {
  */
 function _system_rebuild_module_data() {
   $listing = new ExtensionDiscovery(\Drupal::root());
-  // Find modules
-  $modules = $listing->scan('module');
 
-  // Find installation profiles.
+  // Find installation profiles. This needs to happen before
+  // performing a module scan as the module scan requires knowing
+  // what the active profile is.
   $profiles = $listing->scan('profile');
+  $profile = drupal_get_profile();
+  if ($profile && isset($profiles[$profile])) {
+    // Prime the drupal_get_filename() static cache with the profile info file
+    // location so we can use drupal_get_path() on the active profile during
+    // the module scan.
+    drupal_get_filename('profile', $profile, $profiles[$profile]->getPathname());
+  }
 
+  // Find modules.
+  $modules = $listing->scan('module');
   // Include the installation profile in modules that are loaded.
-  if ($profile = drupal_get_profile()) {
+  if ($profile) {
     $modules[$profile] = $profiles[$profile];
     // Installation profile hooks are always executed last.
     $modules[$profile]->weight = 1000;
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.