diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
index 924e427..a13fc1b 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
@@ -14,17 +14,19 @@
   define('DRUPAL_ROOT', dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__)))));
 }
 
-define('CSS_AGGREGATE_DEFAULT', 0);
-define('CSS_AGGREGATE_THEME', 100);
-define('CSS_BASE', -200);
-define('CSS_LAYOUT', -100);
-define('CSS_COMPONENT', 0);
-define('CSS_STATE', 100);
-define('CSS_SKIN', 200);
-define('JS_SETTING', -200);
-define('JS_LIBRARY', -100);
-define('JS_DEFAULT', 0);
-define('JS_THEME', 100);
+if (!defined('CSS_AGGREGATE_DEFAULT')) {
+  define('CSS_AGGREGATE_DEFAULT', 0);
+  define('CSS_AGGREGATE_THEME', 100);
+  define('CSS_BASE', -200);
+  define('CSS_LAYOUT', -100);
+  define('CSS_COMPONENT', 0);
+  define('CSS_STATE', 100);
+  define('CSS_SKIN', 200);
+  define('JS_SETTING', -200);
+  define('JS_LIBRARY', -100);
+  define('JS_DEFAULT', 0);
+  define('JS_THEME', 100);
+}
 
 /**
  * Tests the library discovery.