diff -u b/core/modules/system/lib/Drupal/system/Tests/Graph/GraphUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Graph/GraphUnitTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Graph/GraphUnitTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Graph/GraphUnitTest.php
@@ -35,16 +35,14 @@
     //       |     |     |
     //       |     |     |
     //       +---> 4 <-- 7      8 ---> 9
-    $graph = $this->normalizeGraph(array(
-      1 => array(2),
-      2 => array(3, 4),
-      3 => array(),
-      4 => array(3),
-      5 => array(6),
-      7 => array(4, 5),
-      8 => array(9),
-      9 => array(),
-    ));
+    $graph[1]['edges'][2] = 1;
+    $graph[2]['edges'][3] = 1;
+    $graph[2]['edges'][4] = 1;
+    $graph[4]['edges'][3] = 1;
+    $graph[5]['edges'][6] = 1;
+    $graph[7]['edges'][4] = 1;
+    $graph[7]['edges'][5] = 1;
+    $graph[8]['edges'][9] = 1;
     $graph_object = new Graph($graph);
     $graph = $graph_object->searchAndSort();
 
@@ -126,15 +123,0 @@
-   * Return a normalized version of a graph.
-   */
-  function normalizeGraph($graph) {
-    $normalized_graph = array();
-    foreach ($graph as $vertex => $edges) {
-      // Create vertex even if it hasn't any edges.
-      $normalized_graph[$vertex] = array();
-      foreach ($edges as $edge) {
-        $normalized_graph[$vertex]['edges'][$edge] = TRUE;
-      }
-    }
-    return $normalized_graph;
-  }
-
-  /**