diff --git a/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php b/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
index 75282d2..b870b09 100644
--- a/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
+++ b/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
@@ -56,8 +56,11 @@ public function __construct(ContainerBuilder $container)
      * Loads a Yaml file.
      *
      * @param mixed  $file The resource
+     * @param bool
+     *   (optional) Whether a later service definition with the same name should
+     *   override an earlier one.
      */
-    public function load($file)
+    public function load($file, $override = FALSE)
     {
         if (!isset(static::$yaml[$file])) {
           static::$yaml[$file] = $this->loadFile($file);
@@ -88,7 +91,7 @@ public function load($file)
         //$this->loadFromExtensions($content);
 
         // services
-        $this->parseDefinitions($content, $file);
+        $this->parseDefinitions($content, $file, $override);
     }
 
     /**
@@ -97,14 +100,14 @@ public function load($file)
      * @param array  $content
      * @param string $file
      */
-    private function parseDefinitions($content, $file)
+    private function parseDefinitions($content, $file, $override)
     {
         if (!isset($content['services'])) {
             return;
         }
 
         foreach ($content['services'] as $id => $service) {
-            $this->parseDefinition($id, $service, $file);
+            $this->parseDefinition($id, $service, $file, $override);
         }
     }
 
@@ -117,7 +120,7 @@ private function parseDefinitions($content, $file)
      *
      * @throws InvalidArgumentException When tags are invalid
      */
-    private function parseDefinition($id, $service, $file)
+    private function parseDefinition($id, $service, $file, $override)
     {
         if (is_string($service) && 0 === strpos($service, '@')) {
             $this->container->setAlias($id, substr($service, 1));
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 49a4d02..9190134 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -1076,7 +1076,7 @@ protected function compileContainer() {
     }
     // Register site-specific service overrides.
     foreach ($this->serviceYamls['site'] as $filename) {
-      $yaml_loader->load($filename);
+      $yaml_loader->load($filename, TRUE);
     }
     foreach ($this->serviceProviders['site'] as $provider) {
       if ($provider instanceof ServiceProviderInterface) {