diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index f3b4169..05c81ef 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -33,13 +33,6 @@ class TestDiscovery {
   protected $cacheBackend;
 
   /**
-   * Cached ExtensionDiscovery.
-   *
-   * @var \Drupal\Core\Extension\ExtensionDiscovery
-   */
-  protected $extensionDiscovery;
-
-  /**
    * Cached map of all test namespaces to respective directories.
    *
    * @var array
@@ -89,17 +82,19 @@ public function __construct($root, $class_loader, ModuleHandlerInterface $module
   }
 
   /**
-   * Registers test namespaces of all available extensions.
+   * Registers the test namespaces of all available extensions.
    *
    * @return array
    *   An associative array whose keys are PSR-4 namespace prefixes and whose
    *   values are directory names.
+   *
+   * @see https://www.drupal.org/node/2116043
    */
   public function registerTestNamespaces() {
     if (isset($this->testNamespaces)) {
       return $this->testNamespaces;
     }
-    $this->testNamespaces = [];
+    $this->testNamespaces = array();
 
     $existing = $this->classLoader->getPrefixesPsr4();
 
@@ -109,7 +104,7 @@ public function registerTestNamespaces() {
     $this->testNamespaces['Drupal\\FunctionalTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalTests'];
     $this->testNamespaces['Drupal\\FunctionalJavascriptTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalJavascriptTests'];
 
-    $this->availableExtensions = [];
+    $this->availableExtensions = array();
     foreach ($this->getExtensions() as $name => $extension) {
       $this->availableExtensions[$extension->getType()][$name] = $name;
 
@@ -126,9 +121,11 @@ public function registerTestNamespaces() {
       // Add PHPUnit-based test namespaces.
       $this->testNamespaces["Drupal\\Tests\\$name\\"][] = "$base_path/tests/src";
     }
+
     foreach ($this->testNamespaces as $prefix => $paths) {
       $this->classLoader->addPsr4($prefix, $paths);
     }
+
     return $this->testNamespaces;
   }
 
@@ -158,6 +155,7 @@ public function registerTestNamespaces() {
   public function getTestClasses($extension = NULL, array $types = []) {
     $reader = new SimpleAnnotationReader();
     $reader->addNamespace('Drupal\\simpletest\\Annotation');
+
     if (!isset($extension)) {
       if ($this->cacheBackend && $cache = $this->cacheBackend->get('simpletest:discovery:classes')) {
         return $cache->data;
@@ -465,17 +463,21 @@ public static function getTestSuite($classname) {
         return 'Simpletest';
       }
     }
-    throw new TestInWrongNamespaceException('Test class ' . $classname . ' is in the wrong place.');
+    throw new TestInWrongNamespaceException(' -- Test class ' . $classname . ' cannot be assigned to a test suite. See https://www.drupal.org/phpunit. -- ');
   }
 
   /**
    * Determines the phpunit testsuite for a given classname.
    *
+   * The current PHPUnit test suites are: Unit, Kernel, Functional, and
+   * FunctionalJavascript.
+   *
    * @param string $classname
    *   The test classname.
    *
    * @return string|false
-   *   The testsuite name or FALSE if its not a phpunit test.
+   *   The testsuite name or FALSE if the class name does not reflect one of
+   *   Unit, Kernel, Functional, or FunctionalJavascript.
    */
   public static function getPhpunitTestSuite($classname) {
     if (preg_match('/Drupal\\\\Tests\\\\Core\\\\(\w+)/', $classname, $matches)) {
@@ -510,14 +512,12 @@ public static function getPhpunitTestSuite($classname) {
    *   An array of Extension objects, keyed by extension name.
    */
   protected function getExtensions() {
-    if (empty($this->extensionDiscovery)) {
-      $this->extensionDiscovery = new ExtensionDiscovery($this->root);
-      // Ensure that tests in all profiles are discovered.
-      $this->extensionDiscovery->setProfileDirectories([]);
-    }
-    $extensions = $this->extensionDiscovery->scan('module', TRUE);
-    $extensions += $this->extensionDiscovery->scan('profile', TRUE);
-    $extensions += $this->extensionDiscovery->scan('theme', TRUE);
+    $listing = new ExtensionDiscovery($this->root);
+    // Ensure that tests in all profiles are discovered.
+    $listing->setProfileDirectories(array());
+    $extensions = $listing->scan('module', TRUE);
+    $extensions += $listing->scan('profile', TRUE);
+    $extensions += $listing->scan('theme', TRUE);
     return $extensions;
   }