Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/page_cache/tests/modules/page_cache_form_test.info.yml

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+name: 'Page Cache Form Test'
+type: module
+description: 'Support module for the Page Cache module tests.'
+core: 8.x
+package: Testing
+version: VERSION

core/modules/page_cache/tests/modules/page_cache_form_test.install

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+<?php
+
+/**
+ * @file
+ *
+ * Install hooks for page_cache_form_test.
+ */
+
+function page_cache_form_test_install() {
+  // Set an explicit module weight, to ensure that the form alter hook is
+  // always called after page_cache_form_alter().
+  module_set_weight('page_cache_form_test', 10);
+}

core/modules/page_cache/tests/modules/page_cache_form_test.module

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+<?php
+
+/**
+ * @file
+ * Provides functionality for testing form caching.
+ */
+
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+* Implements hook_form_FORM_ID_alter().
+*/
+function page_cache_form_test_form_page_cache_form_test_alter(&$form, FormStateInterface $form_state, $form_id) {
+  if (isset($form_state->getBuildInfo()['immutable']) && $form_state->getBuildInfo()['immutable']) {
+    $form['#suffix'] = 'Immutable: TRUE';
+  }
+  else {
+    $form['#suffix'] = 'Immutable: FALSE';
+  }
+}

core/modules/page_cache/tests/modules/page_cache_form_test.routing.yml

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+page_cache_form_test.test_immutability:
+  path: '/page_cache_form_test_immutability'
+  defaults:
+    _form: '\Drupal\page_cache_form_test\Form\TestForm'
+  requirements:
+    _access: 'TRUE'

core/modules/page_cache/tests/modules/src/Form/TestForm.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\page_cache_form_test\Form\TestForm.
+ */
+
+namespace Drupal\page_cache_form_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+class TestForm extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'page_cache_form_test';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $form['#prefix'] = '<p>Llamas are awesome, but kittens are pretty cool too!</p>';
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) { }
+
+}