diff --git a/core/tests/Drupal/Tests/Component/Plugin/Factory/ReflectionFactoryTest.php b/core/tests/Drupal/Tests/Component/Plugin/Factory/ReflectionFactoryTest.php
index 5d5fe63..d6cc8c6 100644
--- a/core/tests/Drupal/Tests/Component/Plugin/Factory/ReflectionFactoryTest.php
+++ b/core/tests/Drupal/Tests/Component/Plugin/Factory/ReflectionFactoryTest.php
@@ -137,6 +137,8 @@ public function testGetInstanceArguments($expected, $reflector_name, $plugin_id,
 
 /**
  * Override ReflectionFactory because ::createInstance() calls a static method.
+ *
+ * We have to override getPluginClass so that we can stub out its return value.
  */
 class StubReflectionFactory extends ReflectionFactory {
 
@@ -144,11 +146,16 @@ class StubReflectionFactory extends ReflectionFactory {
    * {@inheritdoc}
    */
   public static function getPluginClass($plugin_id, $plugin_definition = NULL, $required_interface = NULL) {
-    $class = $plugin_definition["$plugin_id"]['class'];
-    return $class;
+    // Return the class name from the plugin definition.
+    return $plugin_definition[$plugin_id]['class'];
   }
 }
 
+/**
+ * A stub class used by testGetInstanceArguments().
+ *
+ * @see providerGetInstanceArguments()
+ */
 class ArgumentsPluginId {
 
   public function __construct($plugin_id) {
@@ -157,6 +164,11 @@ public function __construct($plugin_id) {
 
 }
 
+/**
+ * A stub class used by testGetInstanceArguments().
+ *
+ * @see providerGetInstanceArguments()
+ */
 class ArgumentsMany {
 
   public function __construct(
@@ -167,6 +179,11 @@ public function __construct(
 
 }
 
+/**
+ * A stub class used by testGetInstanceArguments().
+ *
+ * @see providerGetInstanceArguments()
+ */
 class ArgumentsConfigArrayKey {
 
   public function __construct($config_name) {
@@ -175,6 +192,11 @@ public function __construct($config_name) {
 
 }
 
+/**
+ * A stub class used by testGetInstanceArguments().
+ *
+ * @see providerGetInstanceArguments()
+ */
 class ArgumentsAllNull {
 
   public function __construct($charismatic, $demure, $delightful, $electrostatic) {
@@ -183,6 +205,11 @@ public function __construct($charismatic, $demure, $delightful, $electrostatic)
 
 }
 
+/**
+ * A stub class used by testGetInstanceArguments().
+ *
+ * @see providerGetInstanceArguments()
+ */
 class ArgumentsNoConstructor {
 
 }