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/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..3622053 100644
--- a/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
+++ b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
@@ -98,8 +98,11 @@ protected function getAllFolders() {
         // 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_folders = $this->getComponentNames('profile', array($profile));
+          $this->folders = $profile_folders + $this->folders;
+        }
       }
     }
     return $this->folders;
diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index 2c2a6a5..1520c5e 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -82,7 +82,7 @@ function _ckeditor_theme_css($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 = $info['ckeditor_stylesheets'];
diff --git a/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php b/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php
index 3cb8ab0..0285432 100644
--- a/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php
+++ b/core/modules/comment/src/Tests/CommentStringIdEntitiesTest.php
@@ -27,7 +27,6 @@ class CommentStringIdEntitiesTest extends KernelTestBase {
     'user',
     'field',
     'field_ui',
-    'entity',
     'entity_test',
     'text',
   );
diff --git a/core/modules/quickedit/quickedit.module b/core/modules/quickedit/quickedit.module
index 6813fe4..38805a2 100644
--- a/core/modules/quickedit/quickedit.module
+++ b/core/modules/quickedit/quickedit.module
@@ -77,7 +77,7 @@ function quickedit_library_info_alter(&$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.