diff --git a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
index 05c6eef5dd..311d75b764 100644
--- a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
+++ b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
@@ -96,6 +96,7 @@ public function register($id, $class = null) {
    */
   public function setAlias($alias, $id) {
     $alias = parent::setAlias($alias, $id);
+    // As of Symfony 3.4 all aliases are private by default.
     $alias->setPublic(TRUE);
     return $alias;
   }
diff --git a/core/tests/Drupal/Tests/Component/EventDispatcher/ContainerAwareEventDispatcherTest.php b/core/tests/Drupal/Tests/Component/EventDispatcher/ContainerAwareEventDispatcherTest.php
index 80c8cedda9..baafcd594b 100644
--- a/core/tests/Drupal/Tests/Component/EventDispatcher/ContainerAwareEventDispatcherTest.php
+++ b/core/tests/Drupal/Tests/Component/EventDispatcher/ContainerAwareEventDispatcherTest.php
@@ -23,7 +23,6 @@
  * @see https://github.com/symfony/symfony/pull/12521
  *
  * @group EventDispatcher
- * @group legacy
  */
 class ContainerAwareEventDispatcherTest extends SymfonyContainerAwareEventDispatcherTest
 {
@@ -196,6 +195,7 @@ public function testGetListenerPriorityWithServices()
 
     /**
      * @expectedDeprecation The Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher class is deprecated since Symfony 3.3 and will be removed in 4.0. Use EventDispatcher with closure factories instead.
+     * @group legacy
      */
     public function testAddAListenerService() {
       parent::testAddAListenerService();
@@ -203,6 +203,7 @@ public function testAddAListenerService() {
 
     /**
      * @expectedDeprecation The Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher class is deprecated since Symfony 3.3 and will be removed in 4.0. Use EventDispatcher with closure factories instead.
+     * @group legacy
      */
     public function testPreventDuplicateListenerService() {
       parent::testPreventDuplicateListenerService();
@@ -210,6 +211,7 @@ public function testPreventDuplicateListenerService() {
 
     /**
      * @expectedDeprecation The Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher class is deprecated since Symfony 3.3 and will be removed in 4.0. Use EventDispatcher with closure factories instead.
+     * @group legacy
      */
     public function testAddASubscriberService() {
       parent::testAddASubscriberService();
@@ -217,6 +219,7 @@ public function testAddASubscriberService() {
 
     /**
      * @expectedDeprecation The Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher class is deprecated since Symfony 3.3 and will be removed in 4.0. Use EventDispatcher with closure factories instead.
+     * @group legacy
      */
     public function testHasListenersOnLazyLoad() {
       parent::testHasListenersOnLazyLoad();
@@ -224,6 +227,7 @@ public function testHasListenersOnLazyLoad() {
 
     /**
      * @expectedDeprecation The Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher class is deprecated since Symfony 3.3 and will be removed in 4.0. Use EventDispatcher with closure factories instead.
+     * @group legacy
      */
     public function testGetListenersOnLazyLoad() {
       parent::testGetListenersOnLazyLoad();
@@ -231,6 +235,7 @@ public function testGetListenersOnLazyLoad() {
 
     /**
      * @expectedDeprecation The Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher class is deprecated since Symfony 3.3 and will be removed in 4.0. Use EventDispatcher with closure factories instead.
+     * @group legacy
      */
     public function testRemoveAfterDispatch() {
       parent::testRemoveAfterDispatch();
@@ -238,6 +243,7 @@ public function testRemoveAfterDispatch() {
 
     /**
      * @expectedDeprecation The Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher class is deprecated since Symfony 3.3 and will be removed in 4.0. Use EventDispatcher with closure factories instead.
+     * @group legacy
      */
     public function testRemoveBeforeDispatch() {
       parent::testRemoveBeforeDispatch();
diff --git a/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php b/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php
index 026154da70..08465d6381 100644
--- a/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php
+++ b/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php
@@ -61,6 +61,25 @@ public function testRegisterException() {
     $container->register('Bar');
   }
 
+  /**
+   * @covers ::register
+   */
+  public function testRegister() {
+    $container = new ContainerBuilder();
+    $service = $container->register('bar');
+    $this->assertTrue($service->isPublic());
+  }
+
+  /**
+   * @covers ::setAlias
+   */
+  public function testSetAlias() {
+    $container = new ContainerBuilder();
+    $container->register('bar');
+    $alias = $container->setAlias('foo', 'bar');
+    $this->assertTrue($alias->isPublic());
+  }
+
   /**
    * Tests serialization.
    */
diff --git a/core/tests/Drupal/Tests/Core/DependencyInjection/YamlFileLoaderTest.php b/core/tests/Drupal/Tests/Core/DependencyInjection/YamlFileLoaderTest.php
index 97b2c942d2..4093b6a2eb 100644
--- a/core/tests/Drupal/Tests/Core/DependencyInjection/YamlFileLoaderTest.php
+++ b/core/tests/Drupal/Tests/Core/DependencyInjection/YamlFileLoaderTest.php
@@ -28,6 +28,9 @@ public function testParseDefinitionsWithProvider() {
 services:
   example_service:
     class: \Drupal\Core\ExampleClass
+  example_private_service:
+    class: \Drupal\Core\ExampleClass
+    public: false
 YAML;
 
     vfsStream::setup('drupal', NULL, [
@@ -39,6 +42,11 @@ class: \Drupal\Core\ExampleClass
     $yaml_file_loader->load('vfs://drupal/modules/example/example.yml');
 
     $this->assertEquals(['_provider' => [['provider' => 'example']]], $builder->getDefinition('example_service')->getTags());
+    $this->assertTrue($builder->getDefinition('example_service')->isPublic());
+    $this->assertFalse($builder->getDefinition('example_private_service')->isPublic());
+    $builder->compile();
+    $this->assertTrue($builder->has('example_service'));
+    $this->assertFalse($builder->has('example_private_service'));
   }
 
 }