diff --git a/core/modules/system/lib/Drupal/system/Tests/Graph/GraphUnitTest.php b/core/tests/Drupal/Tests/Component/Graph/GraphTest.php
similarity index 67%
rename from core/modules/system/lib/Drupal/system/Tests/Graph/GraphUnitTest.php
rename to core/tests/Drupal/Tests/Component/Graph/GraphTest.php
index a68535c..2593a0f 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Graph/GraphUnitTest.php
+++ b/core/tests/Drupal/Tests/Component/Graph/GraphTest.php
@@ -2,20 +2,22 @@
 
 /**
  * @file
- * Definition of Drupal\system\Tests\Graph\GraphUnitTest.
+ * Contains \Drupal\Tests\Component\Graph\GraphTest.
  */
 
-namespace Drupal\system\Tests\Graph;
+namespace Drupal\Tests\Component\Graph;
 
 use Drupal\Component\Graph\Graph;
-use Drupal\simpletest\UnitTestBase;
+use Drupal\Tests\UnitTestCase;
 
 /**
  * Unit tests for the graph handling features.
  *
  * @see Drupal\Component\Graph\Graph
+ * @group Graph
  */
-class GraphUnitTest extends UnitTestBase {
+class GraphTest extends UnitTestCase {
+
   public static function getInfo() {
     return array(
       'name' => 'Directed acyclic graph manipulation',
@@ -27,7 +29,7 @@ public static function getInfo() {
   /**
    * Test depth-first-search features.
    */
-  function testDepthFirstSearch() {
+  public function testDepthFirstSearch() {
     // The sample graph used is:
     // 1 --> 2 --> 3     5 ---> 6
     //       |     ^     ^
@@ -72,7 +74,7 @@ function testDepthFirstSearch() {
     $this->assertReversePaths($graph, $expected_reverse_paths);
 
     // Assert that DFS didn't created "missing" vertexes automatically.
-    $this->assertFALSE(isset($graph[6]), 'Vertex 6 has not been created');
+    $this->assertFalse(isset($graph[6]), 'Vertex 6 has not been created');
 
     $expected_components = array(
       array(1, 2, 3, 4, 5, 7),
@@ -91,9 +93,15 @@ function testDepthFirstSearch() {
   }
 
   /**
-   * Return a normalized version of a graph.
+   * Normalizes a graph.
+   *
+   * @param $graph
+   *   A graph array processed by \Drupal\Component\Graph\Graph::searchAndSort()
+   *
+   * @return array
+   *   The normalized version of a graph.
    */
-  function normalizeGraph($graph) {
+  protected function normalizeGraph($graph) {
     $normalized_graph = array();
     foreach ($graph as $vertex => $edges) {
       // Create vertex even if it hasn't any edges.
@@ -109,17 +117,16 @@ function normalizeGraph($graph) {
    * Verify expected paths in a graph.
    *
    * @param $graph
-   *   A graph array processed by
-   *   Drupal\Component\Graph\Graph::searchAndSort().
+   *   A graph array processed by \Drupal\Component\Graph\Graph::searchAndSort()
    * @param $expected_paths
    *   An associative array containing vertices with their expected paths.
    */
-  function assertPaths($graph, $expected_paths) {
+  protected function assertPaths($graph, $expected_paths) {
     foreach ($expected_paths as $vertex => $paths) {
       // Build an array with keys = $paths and values = TRUE.
       $expected = array_fill_keys($paths, TRUE);
       $result = isset($graph[$vertex]['paths']) ? $graph[$vertex]['paths'] : array();
-      $this->assertEqual($expected, $result, format_string('Expected paths for vertex @vertex: @expected-paths, got @paths', array('@vertex' => $vertex, '@expected-paths' => $this->displayArray($expected, TRUE), '@paths' => $this->displayArray($result, TRUE))));
+      $this->assertEquals($expected, $result, sprintf('Expected paths for vertex %s: %s, got %s', $vertex, $this->displayArray($expected, TRUE), $this->displayArray($result, TRUE)));
     }
   }
 
@@ -127,17 +134,17 @@ function assertPaths($graph, $expected_paths) {
    * Verify expected reverse paths in a graph.
    *
    * @param $graph
-   *   A graph array processed by Drupal\Component\Graph\Graph::searchAndSort().
+   *   A graph array processed by \Drupal\Component\Graph\Graph::searchAndSort()
    * @param $expected_reverse_paths
    *   An associative array containing vertices with their expected reverse
    *   paths.
    */
-  function assertReversePaths($graph, $expected_reverse_paths) {
+  protected function assertReversePaths($graph, $expected_reverse_paths) {
     foreach ($expected_reverse_paths as $vertex => $paths) {
       // Build an array with keys = $paths and values = TRUE.
       $expected = array_fill_keys($paths, TRUE);
       $result = isset($graph[$vertex]['reverse_paths']) ? $graph[$vertex]['reverse_paths'] : array();
-      $this->assertEqual($expected, $result, format_string('Expected reverse paths for vertex @vertex: @expected-paths, got @paths', array('@vertex' => $vertex, '@expected-paths' => $this->displayArray($expected, TRUE), '@paths' => $this->displayArray($result, TRUE))));
+      $this->assertEquals($expected, $result, sprintf('Expected reverse paths for vertex %s: %s, got %s', $vertex, $this->displayArray($expected, TRUE), $this->displayArray($result, TRUE)));
     }
   }
 
@@ -145,11 +152,11 @@ function assertReversePaths($graph, $expected_reverse_paths) {
    * Verify expected components in a graph.
    *
    * @param $graph
-   *   A graph array processed by Drupal\Component\Graph\Graph::searchAndSort().
+   *   A graph array processed by \Drupal\Component\Graph\Graph::searchAndSort().
    * @param $expected_components
    *   An array containing of components defined as a list of their vertices.
    */
-  function assertComponents($graph, $expected_components) {
+  protected function assertComponents($graph, $expected_components) {
     $unassigned_vertices = array_fill_keys(array_keys($graph), TRUE);
     foreach ($expected_components as $component) {
       $result_components = array();
@@ -157,24 +164,24 @@ function assertComponents($graph, $expected_components) {
         $result_components[] = $graph[$vertex]['component'];
         unset($unassigned_vertices[$vertex]);
       }
-      $this->assertEqual(1, count(array_unique($result_components)), format_string('Expected one unique component for vertices @vertices, got @components', array('@vertices' => $this->displayArray($component), '@components' => $this->displayArray($result_components))));
+      $this->assertEquals(1, count(array_unique($result_components)), sprintf('Expected one unique component for vertices %s, got %s', $this->displayArray($component), $this->displayArray($result_components)));
     }
-    $this->assertEqual(array(), $unassigned_vertices, format_string('Vertices not assigned to a component: @vertices', array('@vertices' => $this->displayArray($unassigned_vertices, TRUE))));
+    $this->assertEquals(array(), $unassigned_vertices, sprintf('Vertices not assigned to a component: %s', $this->displayArray($unassigned_vertices, TRUE)));
   }
 
   /**
    * Verify expected order in a graph.
    *
    * @param $graph
-   *   A graph array processed by Drupal\Component\Graph\Graph::searchAndSort().
+   *   A graph array processed by \Drupal\Component\Graph\Graph::searchAndSort()
    * @param $expected_orders
    *   An array containing lists of vertices in their expected order.
    */
-  function assertWeights($graph, $expected_orders) {
+  protected function assertWeights($graph, $expected_orders) {
     foreach ($expected_orders as $order) {
       $previous_vertex = array_shift($order);
       foreach ($order as $vertex) {
-        $this->assertTrue($graph[$previous_vertex]['weight'] < $graph[$vertex]['weight'], format_string('Weights of @previous-vertex and @vertex are correct relative to each other', array('@previous-vertex' => $previous_vertex, '@vertex' => $vertex)));
+        $this->assertTrue($graph[$previous_vertex]['weight'] < $graph[$vertex]['weight'], sprintf('Weights of %s and %s are correct relative to each other', $previous_vertex, $vertex));
       }
     }
   }
@@ -187,7 +194,7 @@ function assertWeights($graph, $expected_orders) {
    * @param $keys
    *   (optional) Whether to output the keys of $paths instead of the values.
    */
-  function displayArray($paths, $keys = FALSE) {
+  protected function displayArray($paths, $keys = FALSE) {
     if (!empty($paths)) {
       return implode(', ', $keys ? array_keys($paths) : $paths);
     }
@@ -195,4 +202,5 @@ function displayArray($paths, $keys = FALSE) {
       return '(empty)';
     }
   }
+
 }