diff --git a/core/lib/Drupal/Core/Extension/ModuleHandler.php b/core/lib/Drupal/Core/Extension/ModuleHandler.php
index 859b270..ae0ba7f 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandler.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandler.php
@@ -19,6 +19,13 @@
 class ModuleHandler implements ModuleHandlerInterface {
 
   /**
+   * Absolute path of the document root directory.
+   *
+   * @var string
+   */
+  protected $root;
+
+  /**
    * List of loaded files.
    *
    * @var array
@@ -85,16 +92,23 @@ class ModuleHandler implements ModuleHandlerInterface {
    *   %container.modules% parameter being set up by DrupalKernel.
    * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
    *   Cache backend for storing module hook implementation information.
+   * @param string $root_directory
+   *   (optional) The absolute path of the document root directory.
    *
    * @see \Drupal\Core\DrupalKernel
    * @see \Drupal\Core\CoreServiceProvider
    */
-  public function __construct(array $module_list = array(), CacheBackendInterface $cache_backend) {
+  public function __construct(array $module_list = array(), CacheBackendInterface $cache_backend, $root_directory = NULL) {
     $this->moduleList = array();
     foreach ($module_list as $name => $module) {
       $this->moduleList[$name] = new Extension($module['type'], $module['pathname'], $module['filename']);
     }
     $this->cacheBackend = $cache_backend;
+
+    if (!isset($root_directory)) {
+      $root_directory = DRUPAL_ROOT;
+    }
+    $this->root = $root_directory;
   }
 
   /**
@@ -183,10 +197,10 @@ public function addProfile($name, $path) {
    */
   protected function add($type, $name, $path) {
     $pathname = "$path/$name.info.yml";
-    if (!file_exists(DRUPAL_ROOT . '/' . $pathname)) {
+    if (!file_exists($this->root . "/$pathname")) {
       throw new \InvalidArgumentException("Unknown $type: $pathname not found.");
     }
-    $filename = file_exists("$path/$name.$type") ? "$name.$type" : NULL;
+    $filename = file_exists($this->root . "/$path/$name.$type") ? "$name.$type" : NULL;
     $this->moduleList[$name] = new Extension($type, $pathname, $filename);
     $this->resetImplementations();
   }
@@ -236,12 +250,12 @@ public function loadAllIncludes($type, $name = NULL) {
   public function loadInclude($module, $type, $name = NULL) {
     if ($type == 'install') {
       // Make sure the installation API is available
-      include_once DRUPAL_ROOT . '/core/includes/install.inc';
+      include_once $this->root . '/core/includes/install.inc';
     }
 
     $name = $name ?: $module;
     if (isset($this->moduleList[$module])) {
-      $file = DRUPAL_ROOT . '/' . $this->moduleList[$module]->getPath() . "/$name.$type";
+      $file = $this->root . '/' . $this->moduleList[$module]->getPath() . "/$name.$type";
       if (is_file($file)) {
         require_once $file;
         return $file;
@@ -684,7 +698,7 @@ public function install(array $module_list, $enable_dependencies = TRUE) {
     }
 
     // Required for module installation checks.
-    include_once DRUPAL_ROOT . '/core/includes/install.inc';
+    include_once $this->root . '/core/includes/install.inc';
 
     /** @var \Drupal\Core\Config\ConfigInstaller $config_installer */
     $config_installer = \Drupal::service('config.installer');
@@ -977,7 +991,7 @@ protected function removeCacheBins($module) {
   public function getModuleDirectories() {
     $dirs = array();
     foreach ($this->getModuleList() as $name => $module) {
-      $dirs[$name] = DRUPAL_ROOT . '/' . $module->getPath();
+      $dirs[$name] = $this->root . '/' . $module->getPath();
     }
     return $dirs;
   }
diff --git a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
index 73e45c5..84e85dc 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
@@ -14,9 +14,9 @@
  * Tests the module handler.
  *
  * @group Drupal
- * @group Module
+ * @group Extension
  *
- * @see \Drupal\Core\Extension\ModuleHandler
+ * @coversDefaultClass \Drupal\Core\Extension\ModuleHandler
  */
 class ModuleHandlerTest extends UnitTestCase {
 
@@ -51,11 +51,14 @@ public static function getInfo() {
   protected function setUp() {
     $this->cacheBackend = $this->getMock('Drupal\Core\Cache\CacheBackendInterface');
 
-    $this->moduleHandler = new ModuleHandler(array(), $this->cacheBackend);
+    $this->moduleHandler = new ModuleHandler(array(), $this->cacheBackend, __DIR__);
   }
 
   /**
    * Tests adding a module.
+   *
+   * @covers ::addModule
+   * @covers ::add
    */
   public function testAddModule() {
     $this->cacheBackend->expects($this->once())
@@ -74,6 +77,9 @@ public function testAddModule() {
    *
    * @expectedException \InvalidArgumentException
    * @expectedExceptionMessage Unknown module: modules/non_existing/name.info.yml not found.
+   *
+   * @covers ::addModule
+   * @covers ::add
    */
   public function testAddModuleNonExisting() {
     $this->moduleHandler->addModule('name', 'modules/non_existing');
@@ -81,6 +87,9 @@ public function testAddModuleNonExisting() {
 
   /**
    * Tests adding a profile.
+   *
+   * @covers ::addProfile
+   * @covers ::add
    */
   public function testAddProfile() {
     $this->cacheBackend->expects($this->once())
@@ -99,12 +108,12 @@ public function testAddProfile() {
    *
    * @expectedException \InvalidArgumentException
    * @expectedExceptionMessage Unknown profile: profiles/non_existing/name.info.yml not found.
+   *
+   * @covers ::addProfile
+   * @covers ::add
    */
   public function testAddProfileNonExisting() {
     $this->moduleHandler->addProfile('name', 'profiles/non_existing');
   }
 
 }
-
-// Explicitly map document root to this test directory.
-define('DRUPAL_ROOT', __DIR__);