diff --git a/core/lib/Drupal/Core/Extension/ExtensionList.php b/core/lib/Drupal/Core/Extension/ExtensionList.php
index 7d393c6..4d5e620 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionList.php
@@ -115,7 +115,16 @@ public function __construct($root, $type, CacheBackendInterface $cache, InfoPars
     $this->cache = $cache;
     $this->infoParser = $info_parser;
     $this->moduleHandler = $module_handler;
-    $this->extensionDiscovery = new ExtensionDiscovery($root);
+    $this->extensionDiscovery = $this->getExtensionDiscovery();
+  }
+
+  /**
+   * Returns the extension discovery.
+   *
+   * @return \Drupal\Core\Extension\ExtensionDiscovery
+   */
+  protected function getExtensionDiscovery() {
+    return new ExtensionDiscovery($this->root);
   }
 
   /**
@@ -174,7 +183,7 @@ public function getName($machine_name) {
    */
   public function getExtension($name) {
     $extensions = $this->listExtensions();
-    if (!isset($extensions[$name])) {
+    if (isset($extensions[$name])) {
       return $extensions[$name];
     }
 
diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
new file mode 100644
index 0000000..f9a9b16
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
@@ -0,0 +1,208 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Extension\ExtensionListTest.
+ */
+
+namespace Drupal\Tests\Core\Extension;
+use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Extension\ExtensionList;
+use Drupal\Core\Extension\InfoParserInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Tests\UnitTestCase;
+use org\bovigo\vfs\vfsStream;
+use org\bovigo\vfs\vfsStreamWrapper;
+use Prophecy\Argument;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Extension\ExtensionList
+ */
+class ExtensionListTest extends UnitTestCase {
+
+  /**
+   * @covers ::getName
+   * @expectedException \InvalidArgumentException
+   */
+  public function testGetNameWithNonExistingExtension() {
+    list($cache, $info_parser, $module_handler) = $this->getMocks();
+    $test_extension_list = new TestExtension($this->root, 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal());
+
+    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
+    $extension_discovery->scan('test_extension')->willReturn([]);
+    $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
+
+    $test_extension_list->getName('test_name');
+  }
+
+  /**
+   * @covers ::getName
+   */
+  public function testGetName() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $this->assertEquals('test name', $test_extension_list->getName('test_name'));
+  }
+
+  /**
+   * @covers ::getExtension
+   * @expectedException \InvalidArgumentException
+   */
+  public function testGetExtensionWithNonExistingExtension() {
+    list($cache, $info_parser, $module_handler) = $this->getMocks();
+    $test_extension_list = new TestExtension($this->root, 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal());
+
+    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
+    $extension_discovery->scan('test_extension')->willReturn([]);
+    $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
+
+    $test_extension_list->getExtension('test_name');
+  }
+
+  /**
+   * @covers ::getExtension
+   */
+  public function testGetExtension() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $extension = $test_extension_list->getExtension('test_name');
+    $this->assertInstanceOf(Extension::class, $extension);
+    $this->assertEquals('test_name', $extension->getName());
+  }
+
+  /**
+   * @covers ::listExtensions
+   */
+  public function testListExtensions() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $extensions = $test_extension_list->listExtensions();
+    $this->assertCount(1, $extensions);
+    $this->assertEquals('test_name', $extensions['test_name']->getName());
+  }
+
+  /**
+   * @covers ::getInfo
+   * @covers ::getAllInfo
+   */
+  public function testGetInfo() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $info = $test_extension_list->getInfo('test_name');
+    $this->assertEquals([
+      'type' => 'test_extension',
+      'core' => '8.x',
+      'name' => 'test name',
+      'mtime' => 123456789,
+    ], $info);
+  }
+
+  /**
+   * @covers ::getAllInfo
+   */
+  public function testGetAllInfo() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $infos = $test_extension_list->getAllInfo();
+    $this->assertEquals(['test_name' => [
+      'type' => 'test_extension',
+      'core' => '8.x',
+      'name' => 'test name',
+      'mtime' => 123456789,
+    ]], $infos);
+  }
+
+  /**
+   * @covers ::getFilenames
+   */
+  public function testGetFilenames() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $filenames = $test_extension_list->getFilenames();
+    $this->assertEquals([
+      'test_name' => 'vfs://drupal_root/example/test_name/test_name.info.yml',
+    ], $filenames);
+  }
+
+  /**
+   * @covers ::getFilename
+   */
+  public function testGetFilename() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $filename = $test_extension_list->getFilename('test_name');
+    $this->assertEquals('vfs://drupal_root/example/test_name/test_name.info.yml', $filename);
+  }
+
+
+  /**
+   * @covers ::setFilename
+   * @covers ::getFilename
+   */
+  public function testSetFilename() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $test_extension_list->setFilename('test_name', 'vfs://drupal_root/example2/test_name/test_name.info.yml');
+    $this->assertEquals('vfs://drupal_root/example2/test_name/test_name.info.yml', $test_extension_list->getFilename('test_name'));
+  }
+
+  /**
+   * @covers ::getPath
+   */
+  public function testGetPath() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $path = $test_extension_list->getPath('test_name');
+    $this->assertEquals('vfs://drupal_root/example/test_name', $path);
+  }
+
+  /**
+   * @return \Drupal\Tests\Core\Extension\TestExtension
+   */
+  protected function setupTestExtensionList() {
+    vfsStream::setup('drupal_root');
+    vfsStream::create([
+      'example' => [
+        'test_name' => [
+          'test_name.info.yml' => Yaml::encode([
+            'name' => 'test name',
+            'type' => 'test_extension',
+            'core' => '8.x',
+          ]),
+        ],
+      ],
+    ]);
+    touch('vfs://drupal_root/example/test_name/test_name.info.yml', 123456789);
+
+    list($cache, $info_parser, $module_handler) = $this->getMocks();
+    $info_parser->parse(Argument::any())->will(function($args) {
+      return Yaml::decode(file_get_contents($args[0]));
+    });
+
+    $test_extension_list = new TestExtension('vfs://drupal_root', 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal());
+
+    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
+    $extension_discovery->scan('test_extension')->willReturn(['test_name' => new Extension($this->root, 'test_extension', 'vfs://drupal_root/example/test_name/test_name.info.yml')]);
+    $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
+    return $test_extension_list;
+  }
+
+  protected function getMocks() {
+    $cache = $this->prophesize(CacheBackendInterface::class);
+    $info_parser = $this->prophesize(InfoParserInterface::class);
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+    return [$cache, $info_parser, $module_handler];
+  }
+
+}
+
+class TestExtension extends ExtensionList {
+
+  public function setExtensionDiscovery(ExtensionDiscovery $extension_discovery) {
+    $this->extensionDiscovery = $extension_discovery;
+  }
+
+}