diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
index 3fb8351..3ec2e96 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
@@ -6,16 +6,20 @@
 use Drupal\Core\Extension\ExtensionDiscovery;
 use Drupal\Tests\UnitTestCase;
 use org\bovigo\vfs\vfsStream;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * Tests discovery of extensions.
  *
+ * @coversDefaultClass \Drupal\Core\Extension\ExtensionDiscovery
  * @group Extension
  */
 class ExtensionDiscoveryTest extends UnitTestCase {
 
   /**
    * Tests extension discovery in a virtual filesystem with vfsStream.
+   *
+   * @covers ::scan
    */
   public function testExtensionDiscoveryVfs() {
 
@@ -26,11 +30,10 @@ public function testExtensionDiscoveryVfs() {
     $vfs = vfsStream::setup('root', NULL, $filesystem);
     $root = $vfs->url();
 
-    static::assertFileExists($root . '/core/modules/system/system.module');
-    static::assertFileExists($root . '/core/modules/system/system.info.yml');
+    $this->assertFileExists($root . '/core/modules/system/system.module');
+    $this->assertFileExists($root . '/core/modules/system/system.info.yml');
 
     // Create an ExtensionDiscovery with $root.
-    # $extension_discovery = new ExtensionDiscovery($root, TRUE, ['no/profile'], 'sites/default');
     $extension_discovery = new ExtensionDiscovery($root, FALSE, NULL, 'sites/default');
 
     /** @var \Drupal\Core\Extension\Extension[][] $extensions_by_type */
@@ -47,17 +50,17 @@ public function testExtensionDiscoveryVfs() {
       }
     }
 
-    static::assertEquals($files_by_type_and_name_expected, $files_by_type_and_name);
+    $this->assertEquals($files_by_type_and_name_expected, $files_by_type_and_name);
 
     $extension_expected = new Extension($root, 'module', 'core/modules/system/system.info.yml', 'system.module');
     $extension_expected->subpath = 'modules/system';
     $extension_expected->origin = 'core';
-    static::assertEquals($extension_expected, $extensions_by_type['module']['system'], 'system');
+    $this->assertEquals($extension_expected, $extensions_by_type['module']['system'], 'system');
 
     $extension_expected = new Extension($root, 'theme_engine', 'core/themes/engines/twig/twig.info.yml', 'twig.engine');
     $extension_expected->subpath = 'themes/engines/twig';
     $extension_expected->origin = 'core';
-    static::assertEquals($extension_expected, $extensions_by_type['theme_engine']['twig'], 'twig');
+    $this->assertEquals($extension_expected, $extensions_by_type['theme_engine']['twig'], 'twig');
   }
 
   /**
@@ -69,8 +72,7 @@ public function testExtensionDiscoveryVfs() {
    *   Format: $[$type][$name] = $yml_file
    *   E.g. $['module']['system'] = 'system.info.yml'
    */
-  private function populateFilesystemStructure(array &$filesystem_structure) {
-
+  protected function populateFilesystemStructure(array &$filesystem_structure) {
     $info_by_file = [
       'core/profiles/standard/standard.info.yml' => [
         'type' => 'profile',
@@ -118,8 +120,7 @@ private function populateFilesystemStructure(array &$filesystem_structure) {
         'core' => '8.x',
       ];
       $type = $info['type'];
-      $content_by_file[$file] = $this->buildYmlContent($info);
-      # vfsStream::newFile($file)->at($vfs)->withContent($yml_content);
+      $content_by_file[$file] = Yaml::dump($info);
       $files_by_type_and_name_expected[$type][$name] = $file;
     }
 
@@ -142,7 +143,7 @@ private function populateFilesystemStructure(array &$filesystem_structure) {
    *   Fragments of the file path.
    * @param string $content
    */
-  private function addFileToFilesystemStructure(array &$filesystem_structure, array $pieces, $content) {
+  protected function addFileToFilesystemStructure(array &$filesystem_structure, array $pieces, $content) {
     $piece = array_shift($pieces);
     if ($pieces !== []) {
       $filesystem_structure += [$piece => []];
@@ -153,19 +154,4 @@ private function addFileToFilesystemStructure(array &$filesystem_structure, arra
     }
   }
 
-  /**
-   * Builds a simple yml file. Nesting is not supported.
-   *
-   * @param string[] $info
-   *
-   * @return string
-   */
-  private function buildYmlContent(array $info) {
-    $content = '';
-    foreach ($info as $key => $value) {
-      $content .= "$key: $value\n";
-    }
-    return $content;
-  }
-
 }