diff --git a/core/lib/Drupal/Core/Form/FormAlter.php b/core/lib/Drupal/Core/Form/FormAlter.php
index ed418ff..49f3062 100644
--- a/core/lib/Drupal/Core/Form/FormAlter.php
+++ b/core/lib/Drupal/Core/Form/FormAlter.php
@@ -48,9 +48,10 @@ public function __construct(ModuleHandlerInterface $module_handler, ThemeManager
   public function alterForm($form_id, &$form, FormStateInterface $form_state) {
     // Invoke hook_form_alter(), hook_form_BASE_FORM_ID_alter(), and
     // hook_form_FORM_ID_alter() implementations.
-    $hooks = array('form');
-    if (isset($form_state['build_info']['base_form_id'])) {
-      $hooks[] = 'form_' . $form_state['build_info']['base_form_id'];
+    $hooks = ['form'];
+    $build_info = $form_state->getBuildInfo();
+    if (isset($build_info['base_form_id'])) {
+      $hooks[] = 'form_' . $build_info['base_form_id'];
     }
     $hooks[] = 'form_' . $form_id;
     $this->moduleHandler->alter($hooks, $form, $form_state, $form_id);
diff --git a/core/tests/Drupal/Tests/Core/Form/FormAlterTest.php b/core/tests/Drupal/Tests/Core/Form/FormAlterTest.php
index 5e61445..907c7f0 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormAlterTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormAlterTest.php
@@ -56,7 +56,7 @@ protected function setUp() {
    */
   public function testAlterForm($form_id, $expected_hooks, $form_state_additions = []) {
     $form = [];
-    $form_state = new FormState($form_state_additions);
+    $form_state = (new FormState())->setFormState($form_state_additions);
 
     $this->moduleHandler->expects($this->once())
       ->method('alter')
@@ -73,14 +73,14 @@ public function testAlterForm($form_id, $expected_hooks, $form_state_additions =
    */
   public function providerTestAlterForm() {
     $data = [];
-    $data[] = [
+    $data['no_build_info'] = [
       'red',
       [
         'form',
         'form_red',
       ],
     ];
-    $data[] = [
+    $data['with_build_info'] = [
       'red',
       [
         'form',
diff --git a/core/tests/Drupal/Tests/Core/Form/FormTestBase.php b/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
index c91a11e..612b35d 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
@@ -140,6 +140,8 @@
    * {@inheritdoc}
    */
   protected function setUp() {
+    parent::setUp();
+
     $this->formCache = $this->getMock('Drupal\Core\Form\FormCacheInterface');
     $this->formAlter = $this->getMock('Drupal\Core\Form\FormAlterInterface');
     $this->urlGenerator = $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface');