diff --git a/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php
index 34a0c72..be2051a 100644
--- a/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php
+++ b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php
@@ -24,10 +24,9 @@ class RecursiveExtensionFilterIterator extends \RecursiveFilterIterator {
     // Root subdirectories.
     'includes',
     'scripts',
-    // @todo
-//    'sites',
     // @todo Skip ./config directories (but not modules/config).
-//    'config', // 'schema',
+    //'config',
+    //'schema',
   );
 
   protected $acceptTests = FALSE;
@@ -38,7 +37,6 @@ public function acceptTests($flag = NULL) {
     }
     if (!$this->acceptTests) {
       $this->skipDirs[] = 'tests';
-      $this->skipDirs[] = 'testing';
     }
   }
 
@@ -49,17 +47,15 @@ public function getChildren() {
   }
 
   public function accept() {
+    // Should normally never be TRUE, since ExtensionDiscovery passes the
+    // FilesystemIterator::SKIP_DOTS flag to RecursiveDirectoryIterator, but
+    // in case this filter is instantiated from elsewhere, ensure that we are
+    // not recursing into parent directories.
     if ($this->isDot()) {
       return FALSE;
     }
     if ($this->isDir()) {
-//      if ($name[0] == '.') {
-//        return FALSE;
-//      }
-      if (in_array($this->current()->getFilename(), $this->skipDirs, TRUE)) {
-        return FALSE;
-      }
-      return TRUE;
+      return !in_array($this->current()->getFilename(), $this->skipDirs, TRUE);
     }
     else {
       return substr($this->current()->getFilename(), -9) == '.info.yml';
diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index ce5d80e..bb859b2 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -71,7 +71,7 @@ class ExtensionDiscovery {
    *   extensions of type 'module'.
    * @param bool $include_tests
    *   (optional) Whether to include test extensions. Defaults to FALSE; i.e.,
-   *   all 'tests' and 'testing' directories are excluded in the search.
+   *   all 'tests' directories are excluded in the search.
    *
    * @return RecursiveDirectoryIterator[]
    *   An associative array of RecursiveDirectoryIterator (SplFileInfo) objects,
@@ -117,19 +117,21 @@ public function scan($directory, $include_tests = FALSE) {
     $searchdirs[] = 'sites/all/' . $directory;
     $searchdirs[] = "$site/$directory";
 
+    // Unless explicitly requested by Simpletest, manually check whether we are
+    // are in a test environment, in which case test extensions must be included.
+    if (!$include_tests) {
+      // @todo Replace with drupal_valid_test_ua() once avaiable.
+      // @see https://drupal.org/node/2171683
+      $include_tests = !empty($GLOBALS['drupal_test_info']['test_run_id']);
+    }
+
     $files = array();
     foreach ($searchdirs as $dir) {
-      if ($include_tests) {
-        $directory_extensions = $this->scanDirectory($dir, $include_tests);
-      }
-      else {
-        if (!isset(static::$files[$dir])) {
-          static::$files[$dir] = $this->scanDirectory($dir);
-        }
-        $directory_extensions = static::$files[$dir];
+      if (!isset(static::$files[$dir][$include_tests])) {
+        static::$files[$dir][$include_tests] = $this->scanDirectory($dir, $include_tests);
       }
-      if (isset($directory_extensions[$type])) {
-        $files = array_merge($files, $this->process($files, $directory_extensions[$type]));
+      if (isset(static::$files[$dir][$include_tests][$type])) {
+        $files = array_merge($files, $this->process($files, static::$files[$dir][$include_tests][$type]));
       }
     }
     return $files;
@@ -213,21 +215,15 @@ protected function process(array $files, array $files_to_add) {
    * Recursively scans a base directory for the requested extension type.
    *
    * @param string $dir
-   *   The base directory or URI to scan, without trailing slash.
-   * @param string $key
-   *   The key to be used for the returned associative array of files. 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.
-   * @param string $mask
-   *   The preg_match() regular expression of the files to find.
-   * @param string $nomask
-   *   The preg_match() regular expression of the files to ignore.
+   *   The base directory to scan, without trailing slash.
+   * @param bool $include_tests
+   *   (optional) Whether to include test extensions. Defaults to FALSE; i.e.,
+   *   all 'tests' directories are excluded in the search.
    *
    * @return array
-   *   An associative array (keyed on the chosen key) of objects with 'uri',
-   *   'filename', and 'name' members corresponding to the matching files.
+   *   An associative array whose keys are extension types and whose values are
+   *   associative arrays of RecursiveDirectoryIterator (SplFileInfo) objects,
+   *   keyed by extension name.
    */
   protected function scanDirectory($dir, $include_tests = FALSE) {
     $files = array();