diff --git a/core/lib/Drupal/Core/Render/Element.php b/core/lib/Drupal/Core/Render/Element.php
index ef5c696..c3d2952 100644
--- a/core/lib/Drupal/Core/Render/Element.php
+++ b/core/lib/Drupal/Core/Render/Element.php
@@ -89,7 +89,7 @@ public static function children(array &$elements, $sort = FALSE) {
     }
     // Sort the children if necessary.
     if ($sort && $sortable) {
-      uasort($children, 'element_sort');
+      uasort($children, 'Drupal\Component\Utility\SortArray::sortByWeightProperty');
       // Put the sorted children back into $elements in the correct order, to
       // preserve sorting if the same element is passed through
       // element_children() twice.
diff --git a/core/tests/Drupal/Tests/Core/Render/ElementTest.php b/core/tests/Drupal/Tests/Core/Render/ElementTest.php
index 6148589..2f55339 100644
--- a/core/tests/Drupal/Tests/Core/Render/ElementTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/ElementTest.php
@@ -5,7 +5,7 @@
  * Contains \Drupal\Tests\Core\Render\ElementTest.
  */
 
-namespace Drupal\Tests\Core\Render;
+namespace Drupal\Tests\Core\Render {
 
 use Drupal\Tests\UnitTestCase;
 use Drupal\Core\Render\Element;
@@ -67,7 +67,103 @@ public function testChild() {
    * Tests the children() method.
    */
   public function testChildren() {
+    $element = array(
+      'child2' => array('#weight' => 10),
+      'child1' => array('#weight' => 0),
+      'child3' => array('#weight' => 20),
+      '#property' => 'property',
+    );
+
+    $expected = array('child2', 'child1', 'child3');
+    $element_copy = $element;
+    $this->assertSame($expected, Element::children($element_copy));
+
+    // If #sorted is already set, no sorting should happen.
+    $element_copy = $element;
+    $element_copy['#sorted'] = TRUE;
+    $expected = array('child2', 'child1', 'child3');
+    $this->assertSame($expected, Element::children($element_copy, TRUE));
+
+    // Test with weight sorting, #sorted property should be added.
+    $expected = array('child1', 'child2', 'child3');
+    $element_copy = $element;
+    $this->assertSame($expected, Element::children($element_copy, TRUE));
+    $this->assertArrayHasKey('#sorted', $element_copy);
+    $this->assertTrue($element_copy['#sorted']);
+
+    // The order should stay the same if no weights present.
+    $element_no_weight = array(
+      'child2' => array(),
+      'child1' => array(),
+      'child3' => array(),
+      '#property' => 'property',
+    );
+
+    $expected = array('child2', 'child1', 'child3');
+    $this->assertSame($expected, Element::children($element_no_weight, TRUE));
+  }
+
+  /**
+   * Tests the visibleChildren() method.
+   *
+   * @param array $element
+   *   The test element array.
+   * @param array $expected_keys
+   *   The expected keys to be returned from Element::getVisibleChildren().
+   *
+   * @dataProvider providerVisibleChildren
+   */
+  public function testVisibleChildren(array $element, array $expected_keys) {
+    $this->assertSame($expected_keys, Element::getVisibleChildren($element));
+  }
+
+  /**
+   * Data provider for testVisibleChildren.
+   *
+   * @return array
+   */
+  public function providerVisibleChildren() {
+    return array(
+      array(array('#property1' => '', '#property2' => array()), array()),
+      array(array('#property1' => '', 'child1' => array()), array('child1')),
+      array(array('#property1' => '', 'child1' => array(), 'child2' => array('#access' => TRUE)), array('child1', 'child2')),
+      array(array('#property1' => '', 'child1' => array(), 'child2' => array('#access' => FALSE)), array('child1')),
+      array(array('#property1' => '', 'child1' => array(), 'child2' => array('#type' => 'textfield')), array('child1', 'child2')),
+      array(array('#property1' => '', 'child1' => array(), 'child2' => array('#type' => 'value')), array('child1')),
+      array(array('#property1' => '', 'child1' => array(), 'child2' => array('#type' => 'hidden')), array('child1')),
+    );
+  }
+
+  /**
+   * Tests the setAttributes() method.
+   *
+   * @dataProvider providerTestSetAttributes
+   */
+  public function testSetAttributes($element, $map, $expected_element) {
+    Element::setAttributes($element, $map);
+    $this->assertSame($expected_element, $element);
+  }
 
+  /**
+   * Data provider for testSetAttributes().
+   */
+  public function providerTestSetAttributes() {
+    $base = array('#id' => 'id', '#class' => array());
+    return array(
+      array($base, array(), $base),
+      array($base, array('id', 'class'), $base + array('#attributes' => array('id' => 'id', 'class' => array()))),
+      array($base + array('#attributes' => array('id' => 'id-not-overwritten')), array('id', 'class'), $base + array('#attributes' => array('id' => 'id-not-overwritten', 'class' => array()))),
+    );
   }
 
 }
+
+}
+
+namespace {
+  if (!function_exists('t')) {
+    function t($string) {
+      return $string;
+    }
+  }
+}