diff -u b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
--- b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
+++ b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
@@ -74,20 +74,10 @@
    * Tests non-existing libraries.
    */
   function testLibraryUnknown() {
-    // Set a custom error handler so we can ignore the file not found error.
-    set_error_handler(function($severity, $message, $file, $line) {
-      // Skip error handling if this is a "file not found" error.
-      if (!(error_reporting() & $severity) || strstr($message, 'is missing from the file system:')) {
-        return;
-      }
-      throw new ErrorException($message, 0, $severity, $file, $line);
-    });
-    $build['#attached']['library'][] = 'unknown/unknown';
+    $build['#attached']['library'][] = 'core/unknown';
     $assets = AttachedAssets::createFromRenderArray($build);
 
     $this->assertIdentical([], $this->assetResolver->getJsAssets($assets, FALSE)[0], 'Unknown library was not added to the page.');
-    // Restore the original error handler.
-    restore_error_handler();
   }
 
   /**