diff --git a/core/lib/Drupal/Component/FileSystem/CircularReferenceResistantPathCollection.php b/core/lib/Drupal/Component/FileSystem/CircularReferenceResistantPathCollection.php
new file mode 100644
index 0000000000..ec6d90dee8
--- /dev/null
+++ b/core/lib/Drupal/Component/FileSystem/CircularReferenceResistantPathCollection.php
@@ -0,0 +1,61 @@
+<?php
+
+namespace Drupal\Component\FileSystem;
+
+/**
+ * A collection of paths that have already been iterated over.
+ *
+ * @see \Drupal\Component\FileSystem\CircularReferenceResistantRecursiveDirectoryIterator
+ */
+class CircularReferenceResistantPathCollection {
+
+  /**
+   * The canonical paths of the files in the directory.
+   *
+   * @var string[]
+   *   An associative array of canonical paths of files in the directory that
+   *   have already been iterated over, keyed by realpath. If multiple paths
+   *   exist for a given realpath then the first path that was encountered will
+   *   be considered the canonical path.
+   */
+  protected $canonicalPaths = [];
+
+  /**
+   * Stores a canonical path for a given realpath.
+   *
+   * @param string $realpath
+   *   The realpath to store.
+   * @param string $pathname
+   *   The canonical path name to store.
+   */
+  public function setPath($realpath, $pathname) {
+    $this->canonicalPaths[$realpath] = $pathname;
+  }
+
+  /**
+   * Checks whether we are already tracking the given realpath.
+   *
+   * @param string $realpath
+   *   The realpath to check.
+   *
+   * @return bool
+   *   TRUE if the realpath is already being tracked.
+   */
+  public function realPathExists($realpath) {
+    return array_key_exists($realpath, $this->canonicalPaths);
+  }
+
+  /**
+   * Checks whether we are already tracking the given canonical path.
+   *
+   * @param string $pathname
+   *   The canonical path name to check.
+   *
+   * @return bool
+   *   TRUE if the canonical path name is already being tracked.
+   */
+  public function canonicalPathExists($pathname) {
+    return in_array($pathname, $this->canonicalPaths);
+  }
+
+}
diff --git a/core/lib/Drupal/Component/FileSystem/CircularReferenceResistantRecursiveDirectoryIterator.php b/core/lib/Drupal/Component/FileSystem/CircularReferenceResistantRecursiveDirectoryIterator.php
index 7b3c2255b5..62735a90ae 100644
--- a/core/lib/Drupal/Component/FileSystem/CircularReferenceResistantRecursiveDirectoryIterator.php
+++ b/core/lib/Drupal/Component/FileSystem/CircularReferenceResistantRecursiveDirectoryIterator.php
@@ -2,21 +2,39 @@
 
 namespace Drupal\Component\FileSystem;
 
+/**
+ * A directory iterator that will not follow circular references more than once.
+ */
 class CircularReferenceResistantRecursiveDirectoryIterator extends \RecursiveDirectoryIterator {
 
   /**
-   * @var CircularReferenceResistantRecursiveDirectoryIteratorCanonicalPathCollection
+   * A collection of paths that have already been iterated over.
+   *
+   * @var CircularReferenceResistantPathCollection
    */
   protected $canonicalPathsCollection;
 
-  public function __construct($path, $flags, $canonicalPathsCollection = NULL) {
+  /**
+   * Constructs a new CircularReferenceResistantRecursiveDirectoryIterator.
+   *
+   * @param string $path
+   *   The path to the directory to iterate over.
+   * @param int $flags
+   *   The flags to pass, identical to those of \RecursiveDirectoryIterator
+   * @param \Drupal\Component\FileSystem\CircularReferenceResistantPathCollection $canonicalPathsCollection
+   *   The collection of paths that have already been iterated over.
+   */
+  public function __construct($path, $flags, CircularReferenceResistantPathCollection $canonicalPathsCollection = NULL) {
     if (empty($canonicalPathsCollection)) {
-      $canonicalPathsCollection = new CircularReferenceResistantRecursiveDirectoryIteratorCanonicalPathCollection();
+      $canonicalPathsCollection = new CircularReferenceResistantPathCollection();
     }
     $this->canonicalPathsCollection = $canonicalPathsCollection;
     parent::__construct($path, $flags);
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function hasChildren($allow_links = NULL) {
     if ($this->isCanonicalPath()) {
       return parent::hasChildren($allow_links);
@@ -24,6 +42,9 @@ public function hasChildren($allow_links = NULL) {
     return FALSE;
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function getChildren() {
     if ($this->isCanonicalPath()) {
       return new CircularReferenceResistantRecursiveDirectoryIterator($this->getPathname(), $this->getFlags(), $this->canonicalPathsCollection);
@@ -31,6 +52,14 @@ public function getChildren() {
     return new \RecursiveIteratorIterator([]);
   }
 
+  /**
+   * Checks whether the current path is a canonical path.
+   *
+   * @return bool
+   *   Will return TRUE for the first path that references a particular
+   *   dereferenced ("real") path. For all other paths referencing the same
+   *   realpath this will return FALSE.
+   */
   protected function isCanonicalPath() {
     // If we are not following symlinks then all paths will be canonical since
     // there won't be any circular references.
@@ -63,29 +92,3 @@ protected function followingSymlinks() {
   }
 
 }
-
-class CircularReferenceResistantRecursiveDirectoryIteratorCanonicalPathCollection {
-
-  /**
-   * The canonical paths of the files in the directory.
-   *
-   * @var string[]
-   *   An associative array of canonical paths of files in the directory that
-   *   have already been iterated over, keyed by realpath. If multiple paths
-   *   exist for a given realpath then the first path that was encountered will
-   *   be considered the canonical path.
-   */
-  protected $canonicalPaths = [];
-
-  public function setPath($realpath, $pathname) {
-    $this->canonicalPaths[$realpath] = $pathname;
-  }
-
-  public function realPathExists($realpath) {
-    return array_key_exists($realpath, $this->canonicalPaths);
-  }
-
-  public function canonicalPathExists($pathname) {
-    return in_array($pathname, $this->canonicalPaths);
-  }
-}
diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index 638c9b9d99..c3df50cd60 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -5,6 +5,7 @@
 use Doctrine\Common\Annotations\SimpleAnnotationReader;
 use Doctrine\Common\Reflection\StaticReflectionParser;
 use Drupal\Component\Annotation\Reflection\MockFileFinder;
+use Drupal\Component\FileSystem\CircularReferenceResistantRecursiveDirectoryIterator;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ExtensionDiscovery;
@@ -289,7 +290,7 @@ public static function scanDirectory($namespace_prefix, $path) {
     $flags |= \FilesystemIterator::FOLLOW_SYMLINKS;
     $flags |= \FilesystemIterator::CURRENT_AS_SELF;
 
-    $iterator = new \RecursiveDirectoryIterator($path, $flags);
+    $iterator = new CircularReferenceResistantRecursiveDirectoryIterator($path, $flags);
     $filter = new \RecursiveCallbackFilterIterator($iterator, function ($current, $key, $iterator) {
       if ($iterator->hasChildren()) {
         return TRUE;