diff --git a/core/lib/Drupal/Component/Graph/Graph.php b/core/lib/Drupal/Component/Graph/Graph.php
index 8fc7d21..80fcab5 100644
--- a/core/lib/Drupal/Component/Graph/Graph.php
+++ b/core/lib/Drupal/Component/Graph/Graph.php
@@ -18,6 +18,11 @@ class Graph {
   protected $graph;
 
   /**
+   * Holds the last visit order, the opposite of a topological sort list.
+   */
+ protected $lastVisited;
+
+  /**
    * Instantiates the depth first search object.
    *
    * @param $graph
@@ -78,7 +83,7 @@ public function searchAndSort() {
     // request, and then every 1 weighted etc.
     $component_weights = array();
 
-    foreach ($state['last_visit_order'] as $vertex) {
+    foreach ($this->lastVisited as $vertex) {
       $component = $this->graph[$vertex]['component'];
       if (!isset($component_weights[$component])) {
         $component_weights[$component] = 0;
@@ -136,14 +141,7 @@ protected function depthFirstSearch(&$state, $start, &$component = NULL) {
           unset($state['components'][$component]);
           $component = $new_component;
         }
-        // Only visit existing vertices.
-        if (isset($this->graph[$end])) {
-          // Visit the connected vertex.
-          $this->depthFirstSearch($state, $end, $component);
-
-          // All vertices reachable by $end are also reachable by $start.
-          $this->graph[$start]['paths'] += $this->graph[$end]['paths'];
-        }
+        $this->visit($state, $end, $component);
       }
     }
 
@@ -157,6 +155,30 @@ protected function depthFirstSearch(&$state, $start, &$component = NULL) {
 
     // Record the order of the last visit. This is the reverse of the
     // topological order if the graph is acyclic.
-    $state['last_visit_order'][] = $start;
+    $this->lastVisited[] = $start;
   }
+
+  /**
+   * Visit a vertex.
+   *
+   * @param $state
+   *   An associative array. The key 'last_visit_order' stores a list of the
+   *   vertices visited. The key components stores list of vertices belonging
+   *   to the same the component.
+   * @param $start
+   *   An arbitrary vertex.
+   * @param $component
+   *   The component of the last vertex.
+   */
+  protected function visit($state, $vertex, $component) {
+    // Only visit existing vertices.
+    if (isset($this->graph[$vertex])) {
+      // Visit the connected vertex.
+      $this->depthFirstSearch($state, $vertex, $component);
+
+      // All vertices reachable by $vertex are also reachable by $start.
+      $this->graph[$start]['paths'] += $this->graph[$vertex]['paths'];
+    }
+  }
+
 }
diff --git a/core/lib/Drupal/Component/Graph/TopologicalSort.php b/core/lib/Drupal/Component/Graph/TopologicalSort.php
new file mode 100644
index 0000000..4e5a5a4
--- /dev/null
+++ b/core/lib/Drupal/Component/Graph/TopologicalSort.php
@@ -0,0 +1,24 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\Component\Graph\TopologicalSort.
+ */
+
+namespace Drupal\Component\Graph;
+
+class TopologicalSort extends Graph {
+
+  protected function visit($state, $vertex, $component) {
+    $this->graph += array($vertex => array());
+    parent::visit($state, $vertex, $component);
+  }
+
+  public function getTopologicalSortedList() {
+    if (!isset($this->lastVisited)) {
+      $this->searchAndSort();
+    }
+    return array_reverse($this->lastVisited);
+  }
+
+}