diff --git a/core/lib/Drupal/Component/Graph/Graph.php b/core/lib/Drupal/Component/Graph/Graph.php
index 8fc7d21..6b84cac 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 +141,12 @@ 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);
+        $this->graph += array($end => array('drain' => TRUE));
+        // 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'];
-        }
+        // All vertices reachable by $end are also reachable by $start.
+        $this->graph[$start]['paths'] += $this->graph[$end]['paths'];
       }
     }
 
@@ -157,6 +160,26 @@ 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[] = $start;
+  }
+
+  /**
+   * Calculates Topological Sorted List of Vertices.
+   *
+   * The list is DepthFirst sorted so we can loop over the result like
+   * @code
+   *   foreach ($graph->getTopologicalSortedList() as $module) {
+   *     module_enable($module);
+   *   }
+   * @endcode
+   *
+   * @return array
+   */
+  public function getTopologicalSortedList() {
+    if (!isset($this->lastVisited)) {
+      $this->searchAndSort();
+    }
+    return $this->lastVisited;
   }
+
 }
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index b483e7e..93f5449 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -856,7 +856,7 @@ function system_modules($form, $form_state = array()) {
 
     // If this module requires other modules, add them to the array.
     foreach ($module->requires as $requires => $v) {
-      if (!isset($files[$requires])) {
+      if (!empty($files[$requires]['drain'])) {
         $extra['requires'][$requires] = t('@module (<span class="admin-missing">missing</span>)', array('@module' => drupal_ucfirst($requires)));
         $extra['disabled'] = TRUE;
       }
@@ -1158,7 +1158,7 @@ function system_modules_submit($form, &$form_state) {
       $dependencies = array();
       foreach (array_keys($files[$name]->requires) as $required) {
         if (empty($modules[$required]['enabled'])) {
-          if (isset($files[$required])) {
+          if (empty($files[$required]['drain'])) {
             $dependencies[] = $files[$required]->info['name'];
             $modules[$required]['enabled'] = TRUE;
           }