diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index eac2f97cb1b7..5577be475741 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -938,7 +938,7 @@ function drupal_get_filename($type, $name, $filename = NULL) {
         // extension, not just the file we are currently looking for. This
         // prevents unnecessary scans from being repeated when this function is
         // called more than once in the same page request.
-        $matches = drupal_system_listing("/^" . DRUPAL_PHP_FUNCTION_PATTERN . "\.$extension$/", $dir, 'name', 0);
+        $matches = drupal_system_listing("/^" . DRUPAL_PHP_FUNCTION_PATTERN . "\.$extension$/", $dir);
         foreach ($matches as $matched_name => $file) {
           $files[$type][$matched_name] = $file->uri;
         }
diff --git a/core/includes/common.inc b/core/includes/common.inc
index 540f42bcb940..fdfa9e241fcb 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -5119,17 +5119,17 @@ function drupal_cron_cleanup() {
  * depending on what type of object you are looking for. For instance, if you
  * are looking for modules and call:
  * @code
- * drupal_system_listing("/\.module$/", "modules", 'name', 0);
+ * drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.module$/', 'modules');
  * @endcode
- * the function will search the following directories in this order:
- * - /core/modules.
- * - /core/profiles/testing/modules (only if a test is being run).
- * - /core/profiles/active_profile/modules or /profiles/active_profile/modules.
- * - /modules (standard directory for your contrib/custom modules).
- * - /sites/all/modules (legacy contrib/custom modules directory, see above).
- * - /sites/your_site_dir/modules (for multisite installations).
- * and return information about all of the files ending in .module in those
- * directories.
+ * this function will search:
+ * - the core modules directory; i.e., /core/modules
+ * - the testing profile directory; e.g., /profiles/testing/modules
+ * - your installation profile's directory; e.g., /profiles/standard/modules
+ * - the site-wide modules directory; i.e., /modules
+ * - the all-sites directory; i.e., /sites/all/modules
+ * - the site-specific directory; i.e., /sites/example.com/modules
+ * in that order, and return information about all of the files ending in
+ * .module in those directories.
  *
  * The information is returned in an associative array, which can be keyed on
  * the file name ($key = 'filename'), the file name without the extension ($key
@@ -5140,21 +5140,28 @@ function drupal_cron_cleanup() {
  * files found.
  *
  * @param string $mask
- *   The preg_match() regular expression for the files to find.
+ *   The preg_match() regular expression for the files to find. The expression
+ *   must be anchored and use DRUPAL_PHP_FUNCTION_PATTERN for the file name part
+ *   before the extension, since the results could contain matches that do not
+ *   present valid Drupal extensions otherwise.
  * @param string $directory
  *   The subdirectory name in which the files are found. For example,
- *   'core/modules' will search in sub-directories of the /core/modules
- *   directory, sub-directories of /modules/, etc.
+ *   'modules' will search all 'modules' directories and their sub-directories
+ *   as explained above.
  * @param string $key
- *   The key to be used for the associative array returned. Possible values are
- *   'uri', for the file's URI; 'filename', for the basename of the file; and
- *   'name' for the name of the file without the extension. If you choose 'name'
- *   or 'filename', only the highest-precedence file will be returned.
+ *   (optional) The key to be used for the associative array returned. Possible
+ *   values are:
+ *   - 'uri' for the file's URI.
+ *   - 'filename' for the basename of the file.
+ *   - 'name' for the name of the file without the extension.
+ *   For 'name' and 'filename' only the highest-precedence file is returned.
+ *   Defaults to 'name'.
  * @param int $min_depth
- *   Minimum depth of directories to return files from, relative to each
- *   directory searched. For instance, a minimum depth of 2 would find modules
- *   inside /core/modules/node/tests, but not modules directly in
- *   /core/modules/node.
+ *   (optional) Minimum depth of directories to return files from, relative to
+ *   each directory searched. For instance, a directory of 'modules' and a
+ *   minimum depth of 2 would find modules inside /modules/node/tests, but not
+ *   modules directly in /modules/node. Defaults to 1. The default is sufficient
+ *   and suitable for all extension types known by Drupal core.
  *
  * @return array
  *   An associative array of file objects, keyed on the chosen key. Each element
@@ -5204,7 +5211,14 @@ function drupal_system_listing($mask, $directory, $key = 'name', $min_depth = 1)
     require_once DRUPAL_ROOT . '/core/includes/file.inc';
   }
   foreach ($searchdir as $dir) {
-    $files_to_add = file_scan_directory($dir, $mask, array('key' => $key, 'min_depth' => $min_depth));
+    $files_to_add = file_scan_directory($dir, $mask, array(
+      'key' => $key,
+      'min_depth' => $min_depth,
+      // Do not recurse into ./lib directories; they cannot contain extensions.
+      // We also skip templates, css, and js directories.
+      // @todo Find a way to skip ./config directories (but not modules/config).
+      'nomask' => '/^(CVS|lib|templates|css|js)$/',
+    ));
 
     // Duplicate files found in later search directories take precedence over
     // earlier ones, so we want them to overwrite keys in our resulting
@@ -5216,7 +5230,7 @@ function drupal_system_listing($mask, $directory, $key = 'name', $min_depth = 1)
     foreach (array_intersect_key($files_to_add, $files) as $file_key => $file) {
       // If it has no info file, then we just behave liberally and accept the
       // new resource on the list for merging.
-      if (file_exists($info_file = dirname($file->uri) . '/' . $file->name . '.info')) {
+      if (file_exists($info_file = DRUPAL_ROOT . '/' . dirname($file->uri) . '/' . $file->name . '.info')) {
         // Get the .info file for the module or theme this file belongs to.
         $info = drupal_parse_info_file($info_file);
 
diff --git a/core/includes/file.inc b/core/includes/file.inc
index 16ed6192f711..5d4a2bc25308 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -1432,12 +1432,17 @@ function file_download() {
 function file_scan_directory($dir, $mask, $options = array(), $depth = 0) {
   // Merge in defaults.
   $options += array(
-    'nomask' => '/(\.\.?|CVS)$/',
+    'nomask' => '/^CVS$/',
     'callback' => 0,
     'recurse' => TRUE,
     'key' => 'uri',
     'min_depth' => 0,
   );
+  // Normalize $dir only once.
+  if ($depth == 0) {
+    $dir = file_stream_wrapper_uri_normalize($dir);
+    $dir_has_slash = (substr($dir, -1) === '/');
+  }
 
   $options['key'] = in_array($options['key'], array('uri', 'filename', 'name')) ? $options['key'] : 'uri';
   $files = array();
@@ -1446,10 +1451,15 @@ function file_scan_directory($dir, $mask, $options = array(), $depth = 0) {
   if (is_dir($dir)) {
     if($handle = @opendir($dir)) {
       while (FALSE !== ($filename = readdir($handle))) {
-        if (!preg_match($options['nomask'], $filename) && $filename[0] != '.') {
-          $uri = "$dir/$filename";
-          $uri = file_stream_wrapper_uri_normalize($uri);
-          if (is_dir($uri) && $options['recurse']) {
+        // Skip this file if it matches the nomask or starts with a dot.
+        if ($filename[0] != '.' && !preg_match($options['nomask'], $filename)) {
+          if ($depth == 0 && $dir_has_slash) {
+            $uri = "$dir$filename";
+          }
+          else {
+            $uri = "$dir/$filename";
+          }
+          if ($options['recurse'] && is_dir($uri)) {
             // Give priority to files in this folder by merging them in after
             // any subdirectory files.
             $files = array_merge(file_scan_directory($uri, $mask, $options, $depth + 1), $files);
diff --git a/core/includes/install.inc b/core/includes/install.inc
index ee501ba3a2ce..a2666aec7307 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -384,7 +384,7 @@ function drupal_verify_profile($install_state) {
 
   // Get a list of modules that exist in Drupal's assorted subdirectories.
   $present_modules = array();
-  foreach (drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.module$/', 'modules', 'name', 0) as $present_module) {
+  foreach (drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.module$/', 'modules') as $present_module) {
     $present_modules[] = $present_module->name;
   }
 
diff --git a/core/includes/module.inc b/core/includes/module.inc
index 2db6e9990433..0c1a30b4890c 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -1006,7 +1006,7 @@ function module_invoke_all($hook) {
  * Returns an array of modules required by core.
  */
 function drupal_required_modules() {
-  $files = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.info$/', 'modules', 'name', 0);
+  $files = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.info$/', 'modules');
   $required = array();
 
   // An installation profile is required and one must always be loaded.
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 2388601ad395..0acc8b3537c3 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -385,10 +385,20 @@ function simpletest_test_get_all() {
  * Registers namespaces for disabled modules.
  */
 function simpletest_classloader_register() {
-  $all_data = system_rebuild_module_data();
-  $all_data += system_rebuild_theme_data();
-  foreach ($all_data as $name => $data) {
-    drupal_classloader_register($name, dirname($data->filename));
+  // @see drupal_get_filename()
+  $types = array(
+    'theme_engine' => array('dir' => 'themes/engines', 'extension' => 'engine'),
+    'module' => array('dir' => 'modules', 'extension' => 'module'),
+    'theme' => array('dir' => 'themes', 'extension' => 'info'),
+    'profile' => array('dir' => 'profiles', 'extension' => 'profile'),
+  );
+  foreach ($types as $type => $info) {
+    $matches = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.' . $info['extension'] . '$/', $info['dir']);
+    foreach ($matches as $name => $file) {
+      drupal_classloader_register($name, dirname($file->uri));
+      // While being there, prime drupal_get_filename().
+      drupal_get_filename($type, $name, $file->uri);
+    }
   }
 }
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/GetFilenameUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/GetFilenameUnitTest.php
index ae9a1628626f..32705bf374ec 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/GetFilenameUnitTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/GetFilenameUnitTest.php
@@ -22,14 +22,24 @@ class GetFilenameUnitTest extends UnitTestBase {
     );
   }
 
+  function setUp() {
+    parent::setUp();
+
+    // Remove the keyvalue service definition, since this test wants to verify
+    // the filesystem scan fallback of drupal_get_filename().
+    $this->keyvalue_definition = $this->container->getDefinition('keyvalue');
+    $this->container->removeDefinition('keyvalue');
+  }
+
+  function tearDown() {
+    $this->container->setDefinition('keyvalue', $this->keyvalue_definition);
+    parent::tearDown();
+  }
+
   /**
    * Tests that drupal_get_filename() works when the file is not in database.
    */
   function testDrupalGetFilename() {
-    // Reset the static cache so we can test the "db is not active" code of
-    // drupal_get_filename().
-    drupal_static_reset('drupal_get_filename');
-
     // Retrieving the location of a module.
     $this->assertIdentical(drupal_get_filename('module', 'php'), 'core/modules/php/php.module', 'Retrieve module location.');
 
@@ -50,6 +60,6 @@ class GetFilenameUnitTest extends UnitTestBase {
     // Since there is already a core/scripts directory, drupal_get_filename()
     // will automatically check there for 'script' files, just as it does
     // for (e.g.) 'module' files in core/modules.
-    $this->assertIdentical(drupal_get_filename('script', 'test'), 'core/scripts/test.script', 'Retrieve test script location.');
+    $this->assertIdentical(drupal_get_filename('script', 'test'), 'core/scripts/test/test.script');
   }
 }
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 523a20bee691..ee1f12b70b06 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/SystemListingTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/SystemListingTest.php
@@ -55,7 +55,7 @@ class SystemListingTest extends WebTestBase {
 
     // Now scan the directories and check that the files take precedence as
     // expected.
-    $files = drupal_system_listing('/\.module$/', 'modules', 'name', 1);
+    $files = drupal_system_listing('/\.module$/', 'modules');
     foreach ($expected_directories as $module => $directories) {
       $expected_directory = array_shift($directories);
       $expected_filename = "$expected_directory/$module/$module.module";
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 4670f8819471..0e0077fb2492 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2776,10 +2776,10 @@ function system_get_module_info($property) {
  */
 function _system_rebuild_module_data() {
   // Find modules
-  $modules = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.module$/', 'modules', 'name', 0);
+  $modules = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.module$/', 'modules');
 
   // Find installation profiles.
-  $profiles = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.profile$/', 'profiles', 'name', 0);
+  $profiles = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.profile$/', 'profiles');
 
   // Include the installation profile in modules that are loaded.
   $profile = drupal_get_profile();
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 5aa8b22d0175..5029d13359c9 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -28,7 +28,7 @@ else {
 }
 
 // Bootstrap to perform initial validation or other operations.
-drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
+drupal_bootstrap(DRUPAL_BOOTSTRAP_CODE);
 if (!module_exists('simpletest')) {
   simpletest_script_print_error("The simpletest module must be enabled before this script can run.");
   exit;
diff --git a/core/scripts/test.script b/core/scripts/test/test.script
similarity index 100%
rename from core/scripts/test.script
rename to core/scripts/test/test.script