Updated to follow @schmengler's recommendation

Author: kschroeder

setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php

@@ -33,28 +33,16 @@ class ClassesScanner implements ClassesScannerInterface
      * @param string $generationDirectory
      */
 
-    public function __construct(array $excludePatterns = [], $generationDirectory = null)
+    public function __construct(array $excludePatterns = [], DirectoryList $directoryList = null)
     {
         $this->excludePatterns = $excludePatterns;
-        $this->generationDirectory = $generationDirectory;
-    }
-
-    /**
-     * Retrieves the fully qualified path for var/generation.
-     *
-     * @return string
-     */
-
-    public function getGenerationDirectory()
-    {
-        if ($this->generationDirectory === null) {
+        if (!$directoryList instanceof DirectoryList) {
             $directoryList = ObjectManager::getInstance()->get(DirectoryList::class);
-            /* @var $directoryList DirectoryList */
-            $this->generationDirectory = $directoryList->getPath(DirectoryList::GENERATION);
         }
-        return $this->generationDirectory;
+        $this->generationDirectory = $directoryList->getPath(DirectoryList::GENERATION);
     }
 
+
     /**
      * Adds exclude patterns
      *
@@ -75,8 +63,7 @@ public function addExcludePatterns(array $excludePatterns)
 
     public function isGeneration($path)
     {
-        $generation = $this->getGenerationDirectory();
-        return strpos($path, $generation) === 0;
+        return strpos($path, $this->generationDirectory) === 0;
     }
 
     /**

setup/src/Magento/Setup/Test/Unit/Module/Di/Code/Reader/ClassesScannerTest.php

@@ -5,6 +5,8 @@
  */
 namespace Magento\Setup\Test\Unit\Module\Di\Code\Reader;
 
+use Magento\Framework\App\Filesystem\DirectoryList;
+
 class ClassesScannerTest extends \PHPUnit_Framework_TestCase
 {
     /**
@@ -23,7 +25,11 @@ class ClassesScannerTest extends \PHPUnit_Framework_TestCase
     protected function setUp()
     {
         $this->generation = realpath(__DIR__ . '/../../_files/var/generation');
-        $this->model = new \Magento\Setup\Module\Di\Code\Reader\ClassesScanner([], $this->generation);
+        $mock = $this->getMockBuilder(DirectoryList::class)->disableOriginalConstructor()->setMethods(
+            ['getPath']
+        )->getMock();
+        $mock->method('getPath')->willReturn($this->generation);
+        $this->model = new \Magento\Setup\Module\Di\Code\Reader\ClassesScanner([], $mock);
     }
 
     public function testGetList()