diff --git a/core/modules/system/src/Tests/Common/RenderTest.php b/core/modules/system/src/Tests/Common/RenderTest.php
index 6a0f636..bc9369e 100644
--- a/core/modules/system/src/Tests/Common/RenderTest.php
+++ b/core/modules/system/src/Tests/Common/RenderTest.php
@@ -28,104 +28,6 @@ class RenderTest extends KernelTestBase {
   public static $modules = array('system', 'common_test');
 
   /**
-   * Tests the output drupal_render() for some elementary input values.
-   */
-  function testDrupalRenderBasics() {
-    $types = array(
-      // Test handling of #markup as a fallback for #theme hooks.
-      // Theme suggestion is not implemented, #markup should be rendered.
-      array(
-        'name' => '#markup fallback for #theme suggestion not implemented',
-        'value' => array(
-          '#theme' => array('suggestionnotimplemented'),
-          '#markup' => 'foo',
-        ),
-        'expected' => 'foo',
-      ),
-      // Theme suggestion is not implemented, child #markup should be rendered.
-      array(
-        'name' => '#markup fallback for child elements, #theme suggestion not implemented',
-        'value' => array(
-          '#theme' => array('suggestionnotimplemented'),
-          'child' => array(
-            '#markup' => 'foo',
-          ),
-        ),
-        'expected' => 'foo',
-      ),
-      // Theme suggestion is implemented but returns empty string, #markup
-      // should not be rendered.
-      array(
-        'name' => 'Avoid #markup if #theme is implemented but returns an empty string',
-        'value' => array(
-          '#theme' => array('common_test_empty'),
-          '#markup' => 'foo',
-        ),
-        'expected' => '',
-      ),
-      // Theme suggestion is implemented but returns empty string, children
-      // should not be rendered.
-      array(
-        'name' => 'Avoid rendering child elements if #theme is implemented but returns an empty string',
-        'value' => array(
-          '#theme' => array('common_test_empty'),
-          'child' => array(
-            '#markup' => 'foo',
-          ),
-        ),
-        'expected' => '',
-      ),
-
-      // Test handling of #children and child renderable elements.
-      // #theme is implemented so the values of both #children and 'child' will
-      // be ignored - it is the responsibility of the theme hook to render these
-      // if appropriate.
-      array(
-        'name' => '#theme is implemented, #children is set and array has children',
-        'value' => array(
-          '#theme' => 'common_test_foo',
-          '#children' => 'baz',
-          'child' => array('#markup' => 'boo'),
-        ),
-        'expected' => 'foobar',
-      ),
-      // #theme is implemented but #render_children is TRUE. As in the case
-      // where #theme is not set, empty #children means child elements are
-      // rendered recursively.
-      array(
-        'name' => '#theme is implemented, #render_children is TRUE, #children is empty and array has children',
-        'value' => array(
-          '#theme' => 'common_test_foo',
-          '#children' => '',
-          '#render_children' => TRUE,
-          'child' => array(
-            '#markup' => 'boo',
-          ),
-        ),
-        'expected' => 'boo',
-      ),
-      // #theme is implemented but #render_children is TRUE. As in the case
-      // where #theme is not set, #children will take precedence over 'child'.
-      array(
-        'name' => '#theme is implemented, #render_children is TRUE, #children is set and array has children',
-        'value' => array(
-          '#theme' => 'common_test_foo',
-          '#children' => 'baz',
-          '#render_children' => TRUE,
-          'child' => array(
-            '#markup' => 'boo',
-          ),
-        ),
-        'expected' => 'baz',
-      ),
-    );
-
-    foreach($types as $type) {
-      $this->assertIdentical(drupal_render($type['value']), $type['expected'], '"' . $type['name'] . '" input rendered correctly by drupal_render().');
-    }
-  }
-
-  /**
    * Tests #attached functionality in children elements.
    */
   function testDrupalRenderChildrenAttached() {
@@ -169,24 +71,6 @@ function testDrupalRenderChildrenAttached() {
   }
 
   /**
-   * Tests passing arguments to the theme function.
-   */
-  function testDrupalRenderThemeArguments() {
-    $element = array(
-      '#theme' => 'common_test_foo',
-    );
-    // Test that defaults work.
-    $this->assertEqual(drupal_render($element), 'foobar', 'Defaults work');
-    $element = array(
-      '#theme' => 'common_test_foo',
-      '#foo' => $this->randomMachineName(),
-      '#bar' => $this->randomMachineName(),
-    );
-    // Tests that passing arguments to the theme function works.
-    $this->assertEqual(drupal_render($element), $element['#foo'] . $element['#bar'], 'Passing arguments to theme functions works');
-  }
-
-  /**
    * Tests theme preprocess functions being able to attach assets.
    */
   function testDrupalRenderThemePreprocessAttached() {
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererTest.php b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
index 7f3ba29..0698e20 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
@@ -48,6 +48,13 @@ class RendererTest extends UnitTestCase {
    */
   protected $elementInfo;
 
+  protected $defaultThemeVars = [
+    '#cache' => ['tags' => []],
+    '#attached' => [],
+    '#post_render_cache' => [],
+    '#children' => '',
+  ];
+
   /**
    * {@inheritdoc}
    */
@@ -77,19 +84,19 @@ public function providerTestRenderBasic() {
     $data = [];
 
     // Pass a NULL.
-//    $data[] = [NULL, ''];
+    $data[] = [NULL, ''];
     // Pass an empty string.
-//    $data[] = ['', ''];
+    $data[] = ['', ''];
     // Previously printed, see ::renderTwice for a more integration like test.
-//    $data[] = [[
-//      '#markup' => 'foo',
-//      '#printed' => TRUE,
-//      ], ''];
+    $data[] = [[
+      '#markup' => 'foo',
+      '#printed' => TRUE,
+      ], ''];
     // Printed in pre_render.
-//    $data[] = [[
-//      '#markup' => 'foo',
-//      '#pre_render' => [[new TestCallables(), 'preRenderPrinted']]
-//    ], ''];
+    $data[] = [[
+      '#markup' => 'foo',
+      '#pre_render' => [[new TestCallables(), 'preRenderPrinted']]
+    ], ''];
 
     // Test that #theme and #theme_wrappers can co-exist on an element.
     // #theme and #theme_wrappers basic
@@ -185,43 +192,163 @@ public function providerTestRenderBasic() {
     $data[] = [$build, '<div class="foo"></div>' . "\n", $setup_code];
 
     // Test handling of #markup as a fallback for #theme hooks.
+    // Theme suggestion is not implemented, #markup should be rendered.
+    $build = [
+      '#theme' => ['suggestionnotimplemented'],
+      '#markup' => 'foo',
+    ];
+    $setup_code = function() {
+      $this->themeManager->expects($this->once())
+        ->method('render')
+        ->with(['suggestionnotimplemented'], $this->anything())
+        ->willReturn(FALSE);
+    };
+    $data[] = [$build, 'foo', $setup_code];
+
+    // Theme suggestion is not implemented, child #markup should be rendered.
+    $build = [
+      '#theme' => ['suggestionnotimplemented'],
+      'child' => [
+        '#markup' => 'foo',
+      ],
+    ];
+
+    $setup_code = function() {
+      $this->themeManager->expects($this->once())
+        ->method('render')
+        ->with(['suggestionnotimplemented'], $this->anything())
+        ->willReturn(FALSE);
+    };
+
+    $data[] = [$build, 'foo', $setup_code];
+
+    // Theme suggestion is implemented but returns empty string, #markup
+    // should not be rendered.
+    $build = [
+      '#theme' => ['common_test_empty'],
+      '#markup' => 'foo',
+    ];
+
+    $setup_code = function() {
+      $this->themeManager->expects($this->once())
+        ->method('render')
+        ->with(['common_test_empty'], $this->anything())
+        ->willReturn('');
+    };
+
+    $data[] = [$build, '', $setup_code];
+
+    // Theme suggestion is implemented but returns empty string, children
+    // should not be rendered.
+
+    $build = [
+      '#theme' => ['common_test_empty'],
+      'child' => [
+        '#markup' => 'foo',
+      ],
+    ];
+
+    $setup_code = function() {
+      $this->themeManager->expects($this->once())
+        ->method('render')
+        ->with(['common_test_empty'], $this->anything())
+        ->willReturn('');
+    };
+
+    $data[] = [$build, '', $setup_code];
+
+    // Test handling of #children and child renderable elements.
+    // #theme is implemented so the values of both #children and 'child' will
+    // be ignored - it is the responsibility of the theme hook to render these
+    // if appropriate.
+
+    $build = [
+      '#theme' => 'common_test_foo',
+      '#children' => 'baz',
+      'child' => ['#markup' => 'boo'],
+    ];
+
+    $setup_code = function() {
+      $this->themeManager->expects($this->once())
+        ->method('render')
+        ->with('common_test_foo', $this->anything())
+        ->willReturn('foobar');
+    };
+
+    $data[] = [$build, 'foobar', $setup_code];
+
+    // #theme is implemented but #render_children is TRUE. As in the case
+    // where #theme is not set, empty #children means child elements are
+    // rendered recursively.
+    $build = [
+      '#theme' => 'common_test_foo',
+      '#children' => '',
+      '#render_children' => TRUE,
+      'child' => [
+        '#markup' => 'boo',
+      ],
+    ];
+
+    $setup_code = function() {
+      $this->themeManager->expects($this->never())
+        ->method('render');
+    };
+
+    $data[] = [$build, 'boo', $setup_code];
+
+    // #theme is implemented but #render_children is TRUE. As in the case
+    // where #theme is not set, #children will take precedence over 'child'.
+
+    $build = [
+      '#theme' => 'common_test_foo',
+      '#children' => 'baz',
+      '#render_children' => TRUE,
+      'child' => [
+        '#markup' => 'boo',
+      ],
+    ];
+
+    $setup_code = function() {
+      $this->themeManager->expects($this->never())
+        ->method('render');
+    };
+
+    $data[] = [$build, 'baz', $setup_code];
+
+    // Test handling of #markup as a fallback for #theme hooks.
     // Simple #markup with no theme.
 
     // Basic #markup based renderable array.
-//    $data[] = [
-//      ['#markup' => 'foo']
-//    , 'foo'];
+    $data[] = [
+      ['#markup' => 'foo']
+    , 'foo'];
 
     // Test handling of #children and child renderable elements.
     // #theme is not set, #children is not set and the array has children.
-//    $data[] = [
-//      [
-//        'child' => ['#markup' => 'bar'],
-//      ], 'bar'];
+    $data[] = [
+      [
+        'child' => ['#markup' => 'bar'],
+      ], 'bar'];
     // #theme is not set, #children is set but empty and the array has
     // children.
-//    $data[] = [
-//      [
-//        '#children' => '',
-//        'child' => ['#markup' => 'bar'],
-//      ], 'bar'
-//    ];
+    $data[] = [
+      [
+        '#children' => '',
+        'child' => ['#markup' => 'bar'],
+      ], 'bar'
+    ];
     // #theme is not set, #children is not empty and will be assumed to be the
     // rendered child elements even though the #markup for 'child' differs.
-//    $data[] = [
-//      [
-//        '#children' => 'foo',
-//        'child' => ['#markup' => 'bar'],
-//      ], 'foo'
-//    ];
+    $data[] = [
+      [
+        '#children' => 'foo',
+        'child' => ['#markup' => 'bar'],
+      ], 'foo'
+    ];
 
     return $data;
   }
 
-  protected function setupThemeCommonTestFoo() {
-
-  }
-
   /**
    * @covers ::render
    */
@@ -255,6 +382,9 @@ public function testRenderSorting() {
     $this->assertTrue(array_shift($children) == 'second', 'Child found in the correct order.');
   }
 
+  /**
+   * @covers ::render
+   */
   public function testRenderSortingWithSetHashSorted() {
     $first = $this->randomMachineName();
     $second = $this->randomMachineName();
@@ -290,6 +420,7 @@ public function testRenderWithPresetAccess($access) {
 
   /**
    * @dataProvider providerBoolean
+   * @covers ::render
    */
   public function testRenderWithAccessCallbackCallable($access) {
     $build = [
@@ -306,6 +437,8 @@ public function testRenderWithAccessCallbackCallable($access) {
    * @dataProvider providerBoolean
    *
    * Ensure that the #access property wins over the callable.
+   *
+   * @covers ::render
    */
   public function testRenderWithAccessPropertyAndCallback($access) {
     $build = [
@@ -321,6 +454,8 @@ public function testRenderWithAccessPropertyAndCallback($access) {
 
   /**
    * @dataProvider providerBoolean
+   *
+   * @covers ::render
    */
   public function testRenderWithAccessControllerResolved($access) {
     $build = [
@@ -338,6 +473,9 @@ public function testRenderWithAccessControllerResolved($access) {
     $this->assertAccess($build, $access);
   }
 
+  /**
+   * @covers ::render
+   */
   public function testRenderTwice() {
     $build = [
       '#markup' => 'test',
@@ -384,6 +522,44 @@ protected function setupThemeContainerMultiSuggestion($matcher = NULL) {
       });
   }
 
+  /**
+   * @covers ::render
+   */
+  public function testRenderWithoutThemeArguments() {
+    $element = array(
+      '#theme' => 'common_test_foo',
+    );
+
+    $this->themeManager->expects($this->once())
+      ->method('render')
+      ->with('common_test_foo', $this->defaultThemeVars + $element)
+      ->willReturn('foobar');
+
+    // Test that defaults work.
+    $this->assertEquals($this->renderer->render($element), 'foobar', 'Defaults work');
+  }
+
+  /**
+   * @covers ::render
+   */
+  public function testRenderWithThemeArguments() {
+    $element = array(
+      '#theme' => 'common_test_foo',
+      '#foo' => $this->randomMachineName(),
+      '#bar' => $this->randomMachineName(),
+    );
+
+    $this->themeManager->expects($this->once())
+      ->method('render')
+      ->with('common_test_foo', $this->defaultThemeVars + $element)
+      ->willReturnCallback(function ($hook, $vars) {
+        return $vars['#foo'] . $vars['#bar'];
+      });
+
+    // Tests that passing arguments to the theme function works.
+    $this->assertEquals($this->renderer->render($element), $element['#foo'] . $element['#bar'], 'Passing arguments to theme functions works');
+  }
+
 }
 
 class TestAccessClass {