diff --git a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
index 1edaec3..f9cc1d8 100644
--- a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
+++ b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
@@ -95,4 +95,15 @@ public function __sleep() {
     return array_keys(get_object_vars($this));
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getDefinition($id) {
+    $id = strtolower($id);
+    if ($this->hasAlias($id)) {
+      $id = $this->getAlias($id);
+    }
+    return parent::getDefinition($id);
+  }
+
 }
diff --git a/core/modules/simpletest/src/TestServiceProvider.php b/core/modules/simpletest/src/TestServiceProvider.php
index 5a1a398..bf02cd3 100644
--- a/core/modules/simpletest/src/TestServiceProvider.php
+++ b/core/modules/simpletest/src/TestServiceProvider.php
@@ -33,13 +33,13 @@ function register(ContainerBuilder $container) {
    */
   public function alter(ContainerBuilder $container) {
     if (static::$currentTest instanceof KernelTestBase) {
-      // While $container->get() does a recursive resolve, getDefinition() does
-      // not, so do it ourselves.
       foreach (['router.route_provider' => 'RouteProvider'] as $original_id => $class) {
-        for ($id = $original_id; $container->hasAlias($id); $id = (string) $container->getAlias($id));
-        $definition = $container->getDefinition($id);
+        $definition = $container->getDefinition($original_id);
         $definition->clearTag('needs_destruction');
         $container->setDefinition("simpletest.$original_id", $definition);
+
+        // Resolve $original_id to its final $id similar to get().
+        $id = $container->hasAlias($original_id) ? $container->getAlias($original_id) : $original_id;
         $container->setDefinition($id, new Definition('Drupal\simpletest\\' . $class));
       }
     }