diff -u b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
--- b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
@@ -30,0 +31,10 @@
+  requirements:
+    _access: 'TRUE'
+
+theme_test.priority:
+  path: '/theme-test/priority'
+  options:
+    _custom_theme: 'test_theme'
+  defaults:
+    _content: '\Drupal\theme_test\ThemeTestController::testSuggestion'
+    _title: 'Suggestion'
diff -u b/core/modules/system/tests/modules/theme_test/theme_test.services.yml b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
--- b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
@@ -10,0 +11,5 @@
+
+  theme.negotiator.high_priority:
+    class: Drupal\theme_test\Theme\HighPriorityThemeNegotiator
+    tags:
+      - { name: theme_negotiator, priority: 1000 }
only in patch2:
unchanged:
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -119,6 +119,17 @@ function testPreprocessForSuggestions() {
   }
 
   /**
+   * Tests the priority of some theme negotiators.
+   */
+  public function testNegotiatorPriorities() {
+    $this->drupalGet('theme-test/priority');
+
+    // Ensure that the custom theme negotiator was not able to set the theme.
+
+    $this->assertNoText('Theme hook implementor=test_theme_theme_test__suggestion(). Foo=template_preprocess_theme_test', 'Theme hook suggestion ran with data available from a preprocess function for the base hook.');
+  }
+
+  /**
    * Ensure page-front template suggestion is added when on front page.
    */
   function testFrontPageThemeSuggestion() {