diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 75d50c5..64e01e6 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -189,7 +189,7 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
 
     // Inform $form_state about the request method that's building it, so that
     // it can prevent persisting state changes during HTTP methods for which
-    // that is disallowed by HTTP.
+    // that is disallowed by HTTP: GET and HEAD.
     $form_state->setRequestMethod($request->getMethod());
 
     // Initialize the form's user input. The user input should include only the
diff --git a/core/lib/Drupal/Core/Form/FormStateInterface.php b/core/lib/Drupal/Core/Form/FormStateInterface.php
index 8164227..fd46f70 100644
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -747,7 +747,7 @@ public function getLimitValidationErrors();
    *   methods.
    *
    * @see \Drupal\Core\Form\FormState::$method
-   * @see self::setRequestMethod().
+   * @see \Drupal\Core\Form\FormStateInterface::setRequestMethod()
    *
    * @return $this
    */
@@ -760,6 +760,8 @@ public function setMethod($method);
    *   Can be any valid HTTP method, such as GET, POST, HEAD, etc.
    *
    * @return $this
+   *
+   * @see \Drupal\Core\Form\FormStateInterface::setMethod()
    */
   public function setRequestMethod($method);
 
diff --git a/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
index 136ec69..4113263 100644
--- a/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
+++ b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
@@ -35,7 +35,7 @@ protected function getFormBuildId() {
   }
 
   /**
-   * Create a simple form, then POST via AJAX to change to it.
+   * Create a simple form, then submit the form via AJAX to change to it.
    */
   public function testSimpleAJAXFormValue() {
    $this->drupalGet('ajax_forms_test_get_form');
diff --git a/core/modules/system/src/Tests/Form/FormStoragePageCacheTest.php b/core/modules/system/src/Tests/Form/FormStoragePageCacheTest.php
index e0be445..5cad67f 100644
--- a/core/modules/system/src/Tests/Form/FormStoragePageCacheTest.php
+++ b/core/modules/system/src/Tests/Form/FormStoragePageCacheTest.php
@@ -103,7 +103,7 @@ public function testRebuildFormStorageOnCachedPage() {
     // that initial build ID.
     $edit = ['title' => 'something'];
     $this->drupalPostForm(NULL, $edit, 'Rebuild');
-    $this->assertNoText('No old build id', 'There is an old build id on the page.');
+    $this->assertNoText('No old build id', 'There is no old build id on the page.');
     $this->assertNoText($build_id_initial, 'The old build id is not the initial build id.');
     $build_id_first_rebuild = $this->getFormBuildId();
     $this->assertNotEqual($build_id_initial, $build_id_first_rebuild, 'Build id changes on first rebuild.');
diff --git a/core/modules/system/src/Tests/Form/StorageTest.php b/core/modules/system/src/Tests/Form/StorageTest.php
index 8f3873a..831ba25 100644
--- a/core/modules/system/src/Tests/Form/StorageTest.php
+++ b/core/modules/system/src/Tests/Form/StorageTest.php
@@ -73,16 +73,19 @@ function testFormCached() {
 
     // Use form rebuilding triggered by a submit button.
     $this->drupalPostForm(NULL, $edit, 'Continue submit');
+    // The first one is for the building of the form.
     $this->assertText('Form constructions: 2');
+    // The second one is for the rebuilding of the form.
+    $this->assertText('Form constructions: 3');
 
     // Reset the form to the values of the storage, using a form rebuild
     // triggered by button of type button.
     $this->drupalPostForm(NULL, array('title' => 'changed'), 'Reset');
     $this->assertFieldByName('title', 'new', 'Values have been reset.');
-    $this->assertText('Form constructions: 3');
+    $this->assertText('Form constructions: 4');
 
     $this->drupalPostForm(NULL, $edit, 'Save');
-    $this->assertText('Form constructions: 3');
+    $this->assertText('Form constructions: 4');
     $this->assertText('Title: new', 'The form storage has stored the values.');
   }
 
@@ -130,55 +133,6 @@ function testCachedFormStorageValidation() {
   }
 
   /**
-   * Tests a form using form state without using 'storage' to pass data from the
-   * constructor to a submit handler. The data has to persist even when caching
-   * gets activated, what may happen when a modules alter the form and adds
-   * #ajax properties.
-   */
-  function testFormStatePersist() {
-    // Test the form one time with caching activated and one time without.
-    $run_options = array(
-      array(),
-      array('query' => array('cache' => 1)),
-    );
-    foreach ($run_options as $options) {
-      $this->drupalPostForm('form-test/state-persist', array(), t('Submit'), $options);
-      // The submit handler outputs the value in $form_state, assert it's there.
-      $this->assertText('State persisted.');
-
-      // Test it again, but first trigger a validation error, then test.
-      $this->drupalPostForm('form-test/state-persist', array('title' => ''), t('Submit'), $options);
-      $this->assertText(t('!name field is required.', array('!name' => 'title')));
-      // Submit the form again triggering no validation error.
-      $this->drupalPostForm(NULL, array('title' => 'foo'), t('Submit'), $options);
-      $this->assertText('State persisted.');
-
-      // Now post to the rebuilt form and verify it's still there afterwards.
-      $this->drupalPostForm(NULL, array('title' => 'bar'), t('Submit'), $options);
-      $this->assertText('State persisted.');
-    }
-  }
-
-  /**
-   * Verify that the form build-id remains the same when validation errors
-   * occur on a mutable form.
-   */
-  public function testMutableForm() {
-    // Request the form with 'cache' query parameter to enable form caching.
-    $this->drupalGet('form_test/form-storage', ['query' => ['cache' => 1]]);
-    $buildIdFields = $this->xpath('//input[@name="form_build_id"]');
-    $this->assertEqual(count($buildIdFields), 1, 'One form build id field on the page');
-    $buildId = (string) $buildIdFields[0]['value'];
-
-    // Trigger validation error by submitting an empty title.
-    $edit = ['title' => ''];
-    $this->drupalPostForm(NULL, $edit, 'Continue submit');
-
-    // Verify that the build-id did not change.
-    $this->assertFieldByName('form_build_id', $buildId, 'Build id remains the same when form validation fails');
-  }
-
-  /**
    * Verifies that form build-id is regenerated when loading an immutable form
    * from the cache.
    */
diff --git a/core/modules/system/tests/modules/form_test/form_test.module b/core/modules/system/tests/modules/form_test/form_test.module
index d297945..2a970a2 100644
--- a/core/modules/system/tests/modules/form_test/form_test.module
+++ b/core/modules/system/tests/modules/form_test/form_test.module
@@ -76,19 +76,6 @@ function _form_test_tableselect_get_data() {
 }
 
 /**
- * Implements hook_form_FORM_ID_alter().
- *
- * @see form_test_state_persist()
- */
-function form_test_form_form_test_state_persist_alter(&$form, FormStateInterface $form_state) {
-  // Simulate a form alter implementation inserting form elements that enable
-  // caching of the form, e.g. elements having #ajax.
-  if (\Drupal::request()->get('cache')) {
-    $form_state->setCached();
-  }
-}
-
-/**
  * Implements hook_form_FORM_ID_alter() for the registration form.
  */
 function form_test_form_user_register_form_alter(&$form, FormStateInterface $form_state) {
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
index 9510ef4..ac65ee8 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
@@ -82,12 +82,6 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#value' => 'Save',
     );
 
-    if (\Drupal::request()->get('cache')) {
-      // Manually activate caching, so we can test that the storage keeps working
-      // when it's enabled.
-      $form_state->setCached();
-    }
-
     if ($this->getRequest()->get('immutable')) {
       $form_state->addBuildInfo('immutable', TRUE);
     }
@@ -96,6 +90,17 @@ public function buildForm(array $form, FormStateInterface $form_state) {
   }
 
   /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    if (\Drupal::request()->get('cache')) {
+      // Manually activate caching, so we can test that the storage keeps working
+      // when it's enabled.
+      $form_state->setCached();
+    }
+  }
+
+  /**
    * Form element validation handler for 'value' element.
    *
    * Tests updating of cached form storage during validation.