diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index 79ca932..c57ff91 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -67,13 +67,6 @@ class RegistryTest extends UnitTestCase {
   protected $themeManager;
 
   /**
-   * The theme.
-   *
-   * @var \Drupal\Core\Theme\ThemeManagerInterface|\PHPUnit_Framework_MockObject_MockObject
-   */
-  protected $theme;
-
-  /**
    * The list of functions that get_defined_functions() should provide.
    *
    * @var array
@@ -182,34 +175,45 @@ public function testGetRegistryForModule() {
   }
 
   /**
-   * Sets up the mocks needed for testing Registry::postProcessExtension.
+   * @covers ::postProcessExtension
+   *
+   * @dataProvider providerTestPostProcessExtension
+   *
+   * @param array $defined_functions
+   *   An array of functions to be used in place of get_defined_functions().
+   * @param array $hooks
+   *   An array of theme hooks to process.
+   * @param array $expected
+   *   The expected results.
    */
-  public function setUpPostProcessExtension() {
-    $this->theme = $this->getMockBuilder(ActiveTheme::class)
-      ->disableOriginalConstructor()
-      ->getMock();
-    $this->theme->expects($this->atLeastOnce())
-      ->method('getBaseThemes')
-      ->will($this->returnValue([]));
-    $this->theme->expects($this->atLeastOnce())
-      ->method('getName')
-      ->will($this->returnValue('test'));
+  public function testPostProcessExtension($defined_functions, $hooks, $expected) {
+    static::$functions['user'] = $defined_functions;
+
+    $theme = $this->prophesize(ActiveTheme::class);
+    $theme->getBaseThemes()->willReturn([]);
+    $theme->getName()->willReturn('test');
+    $theme->getEngine()->willReturn('twig');
 
     $this->moduleHandler->expects($this->atLeastOnce())
       ->method('getModuleList')
       ->willReturn([]);
+
+    $class = new \ReflectionClass(TestRegistry::class);
+    $reflection_method = $class->getMethod('postProcessExtension');
+    $reflection_method->setAccessible(TRUE);
+    $reflection_method->invokeArgs($this->registry, [&$hooks, $theme->reveal()]);
+
+    $this->assertArrayEquals($hooks, $expected);
   }
 
   /**
-   * Tests that preprocess functions are added when base hook is specified.
-   *
-   * @covers ::postProcessExtension
+   * Provides test data to ::testPostProcessExtension().
    */
-  public function testManualBaseHook() {
-    static::$functions['user'] = ['cat', 'mouse', 'cheese'];
-
-    // Define a test case registry with mock data.
-    $hooks = [
+  public function providerTestPostProcessExtension() {
+    $data = [];
+    $data['manual_base_hook'] = [
+      'defined_functions' => ['cat', 'mouse', 'cheese'],
+      'hooks' => [
         'test_hook' => [
           'preprocess functions' => ['cat', 'mouse'],
         ],
@@ -218,9 +222,8 @@ public function testManualBaseHook() {
           'base hook' => 'test_hook',
           'incomplete preprocess functions' => TRUE,
         ],
-    ];
-
-    $expected = [
+      ],
+      'expected' => [
         'test_hook' => [
           'preprocess functions' => ['cat', 'mouse'],
         ],
@@ -228,27 +231,18 @@ public function testManualBaseHook() {
           'preprocess functions' => ['cat', 'mouse', 'cheese'],
           'base hook' => 'test_hook',
         ],
+      ],
     ];
-
-    $this->setUpPostProcessExtension();
-
-    $class = new \ReflectionClass(TestRegistry::class);
-    $reflection_method = $class->getMethod('postProcessExtension');
-    $reflection_method->setAccessible(TRUE);
-    $reflection_method->invokeArgs($this->registry, [ &$hooks, $this->theme]);
-
-    $this->assertArrayEquals($hooks, $expected);
-  }
-
-  /**
-   * Tests that a suggestion defined in hook_theme gets preprocess functions.
-   *
-   * @covers ::postProcessExtension
-   */
-  public function testManualSuggestion() {
-    $this->setUpPostProcessExtension();
-
-    $hooks = [
+    $data['manual_suggestion'] = [
+      'defined_functions' => [
+        'cat',
+        'mouse',
+        'cheese',
+        'bread',
+        'test_preprocess_test_hook__suggestion',
+        'test_preprocess_test_hook__suggestion__another',
+      ],
+      'hooks' => [
         'test_hook' => [
           'preprocess functions' => ['cat', 'mouse'],
           'template' => 'test-hook',
@@ -261,9 +255,8 @@ public function testManualSuggestion() {
           'preprocess functions' => ['cheese'],
           'incomplete preprocess functions' => TRUE,
         ],
-    ];
-
-    $expected = [
+      ],
+      'expected' => [
         'test_hook' => [
           'preprocess functions' => [
             'cat',
@@ -293,23 +286,9 @@ public function testManualSuggestion() {
           'base hook' => 'test_hook',
           'template' => 'test-hook',
         ],
+      ],
     ];
-
-    static::$functions['user'] = [
-      'cat',
-      'mouse',
-      'cheese',
-      'bread',
-      'test_preprocess_test_hook__suggestion',
-      'test_preprocess_test_hook__suggestion__another',
-    ];
-
-    $class = new \ReflectionClass(TestRegistry::class);
-    $reflection_method = $class->getMethod('postProcessExtension');
-    $reflection_method->setAccessible(TRUE);
-    $reflection_method->invokeArgs($this->registry, [&$hooks, $this->theme]);
-
-    $this->assertArrayEquals($hooks, $expected);
+    return $data;
   }
 
   protected function setupTheme() {