diff -u b/core/tests/Drupal/Tests/Component/ClassFinder/ClassFinderTest.php b/core/tests/Drupal/Tests/Component/ClassFinder/ClassFinderTest.php
--- b/core/tests/Drupal/Tests/Component/ClassFinder/ClassFinderTest.php
+++ b/core/tests/Drupal/Tests/Component/ClassFinder/ClassFinderTest.php
@@ -20,7 +20,8 @@
 
     // The full path is returned therefore only tests with
     // assertStringEndsWith() so the test is portable.
-    $this->assertStringEndsWith('core' . DIRECTORY_SEPARATOR . 'tests' . DIRECTORY_SEPARATOR . 'Drupal' . DIRECTORY_SEPARATOR . 'Tests' . DIRECTORY_SEPARATOR . 'Component' . DIRECTORY_SEPARATOR . 'ClassFinder' . DIRECTORY_SEPARATOR . 'ClassFinderTest.php', $finder->findFile(ClassFinderTest::class));
+    $path = str_replace('/', DIRECTORY_SEPARATOR, 'core/tests/Drupal/Tests/Component/ClassFinder/ClassFinderTest.php');
+    $this->assertStringEndsWith($path, $finder->findFile(ClassFinderTest::class));
     $class = 'Not\\A\\Class';
     $this->assertNull($finder->findFile($class));
 
@@ -30,7 +31,7 @@
     $loader->register();
     $this->assertEquals(__FILE__, $finder->findFile($class));
     // This shouldn't prevent us from finding the original file.
-    $this->assertStringEndsWith('core' . DIRECTORY_SEPARATOR . 'tests' . DIRECTORY_SEPARATOR . 'Drupal' . DIRECTORY_SEPARATOR . 'Tests' . DIRECTORY_SEPARATOR . 'Component' . DIRECTORY_SEPARATOR . 'ClassFinder' . DIRECTORY_SEPARATOR . 'ClassFinderTest.php', $finder->findFile(ClassFinderTest::class));
+    $this->assertStringEndsWith($path, $finder->findFile(ClassFinderTest::class));
 
     // Clean up the additional autoloader after the test.
     $loader->unregister();