core/modules/page_cache/page_cache.module          |  2 +-
 .../modules/page_cache/src/Tests/PageCacheTest.php | 35 +++++++++++++++++++
 .../tests/modules/page_cache_form_test.info.yml    |  6 ++++
 .../page_cache/tests/modules/src/Form/TestForm.php | 35 +++++++++++++++++++
 .../tests/Drupal/Tests/Core/Form/FormCacheTest.php | 39 ----------------------
 5 files changed, 77 insertions(+), 40 deletions(-)

diff --git a/core/modules/page_cache/page_cache.module b/core/modules/page_cache/page_cache.module
index 2c7d01f..6322363 100644
--- a/core/modules/page_cache/page_cache.module
+++ b/core/modules/page_cache/page_cache.module
@@ -21,7 +21,7 @@ function page_cache_help($route_name, RouteMatchInterface $route_match) {
       $output .= '<dl>';
       $output .= '<dt>' . t('Speeding up your site') . '</dt>';
       $output .= '<dd>';
-      $output .= '<p>' . t('Pages requested by anonymous users are stored in a compressed format; depending on your site configuration and the amount of your web traffic tied to anonymous visitors, the caching system may significantly increase the speed of your site.');
+      $output .= '<p>' . t('Pages requested by anonymous users are stored the first time they are requested and then reused; depending on your site configuration and the amount of your web traffic tied to anonymous visitors, the caching system may significantly increase the speed of your site.');
       $output .= '<p>' . t('(For authenticated users, pages need to be assembled for each user individually, but anonymous users all get the exact same version of each page.)') . '</p>';
       $output .= '</dd>';
       $output .= '<dt>' . t('Configuring Internal page cache') . '</dt>';
diff --git a/core/modules/page_cache/src/Tests/PageCacheTest.php b/core/modules/page_cache/src/Tests/PageCacheTest.php
index 1856315..beb56d9 100644
--- a/core/modules/page_cache/src/Tests/PageCacheTest.php
+++ b/core/modules/page_cache/src/Tests/PageCacheTest.php
@@ -9,11 +9,13 @@
 
 use Drupal\Component\Datetime\DateTimePlus;
 use Drupal\Component\Utility\String;
+use Drupal\Core\Form\FormState;
 use Drupal\Core\Routing\RequestContext;
 use Drupal\Core\Url;
 use Drupal\language\Entity\ConfigurableLanguage;
 use Drupal\simpletest\WebTestBase;
 use Drupal\Core\Cache\Cache;
+use Symfony\Component\HttpFoundation\Request;
 
 /**
  * Enables the page cache and tests it with various HTTP requests.
@@ -194,4 +196,37 @@ public function testPageCacheWithoutVaryCookie() {
     $this->assertTrue(strpos($this->drupalGetHeader('Vary'), 'Cookie') === FALSE, 'Vary: Cookie header was not sent.');
   }
 
+  /**
+   * Test the setting of forms to be immutable.
+   */
+  public function testFormImmutability() {
+    // Install the module that provide sthe test form.
+    $this->container->get('module_installer')->install(['page_cache_form_test']);
+
+    $verify_immutable_flag = function($request_method, $immutable_flag_exists) {
+      // Mock a request.
+      $request = Request::create('/');
+      $request->setMethod($request_method);
+      $this->container->get('request_stack')->push($request);
+
+      // Build a form while the page_cache module is enabled, , the
+      $form_state = new FormState();
+      \Drupal::formBuilder()->buildForm('\Drupal\page_cache_form_test\Form\TestForm', $form_state);
+
+      $this->assertIdentical($immutable_flag_exists, array_key_exists('immutable', $form_state->getBuildInfo()));
+      if ($immutable_flag_exists) {
+        $this->assertIdentical($form_state->getBuildInfo()['immutable'], TRUE);
+      }
+    };
+
+    // Verify the immutable flag is being set on cacheable requests.
+    $verify_immutable_flag('POST', FALSE);
+    $verify_immutable_flag('GET', TRUE);
+
+    // Uninstall the page_cache module, verify the flag is never set.
+    $this->container->get('module_installer')->uninstall(['page_cache']);
+    $verify_immutable_flag('POST', FALSE);
+    $verify_immutable_flag('GET', FALSE);
+  }
+
 }
diff --git a/core/modules/page_cache/tests/modules/page_cache_form_test.info.yml b/core/modules/page_cache/tests/modules/page_cache_form_test.info.yml
new file mode 100644
index 0000000..e18bb53
--- /dev/null
+++ b/core/modules/page_cache/tests/modules/page_cache_form_test.info.yml
@@ -0,0 +1,6 @@
+name: 'Page Cache Form Test'
+type: module
+description: 'Support module for the Page Cache module tests.'
+core: 8.x
+package: Testing
+version: VERSION
diff --git a/core/modules/page_cache/tests/modules/src/Form/TestForm.php b/core/modules/page_cache/tests/modules/src/Form/TestForm.php
new file mode 100644
index 0000000..535f2b2
--- /dev/null
+++ b/core/modules/page_cache/tests/modules/src/Form/TestForm.php
@@ -0,0 +1,35 @@
+<?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) { }
+
+}
diff --git a/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php b/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
index c1f33fc..1820932 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
@@ -487,45 +487,6 @@ public function testSetCacheBuildIdMismatch() {
   }
 
   /**
-   * @covers ::setCache
-   *
-   * @todo Move this test to page_cache module.
-   */
-  public function atestSetCacheImmutableForm() {
-    $form_build_id = 'the_form_build_id';
-    $form = [
-      '#form_id' => 'the_form_id',
-    ];
-    $form_state = new FormState();
-
-    $this->formCacheStore->expects($this->once())
-      ->method('setWithExpire')
-      ->with($form_build_id, $form, $this->isType('int'));
-    $form_state_data = $form_state->getCacheableArray();
-    $form_state_data['build_info']['safe_strings'] = [];
-    // Ensure that the form is marked immutable.
-    $form_state_data['build_info']['immutable'] = TRUE;
-    $this->formStateCacheStore->expects($this->once())
-      ->method('setWithExpire')
-      ->with($form_build_id, $form_state_data, $this->isType('int'));
-
-    // Rebuild the FormCache with a config factory that will return a config
-    // object with the page_cache module enabled.
-    $this->configFactory = $this->getConfigFactoryStub(['system.performance' => ['cache.page.use_internal' => TRUE]]);
-    $root = dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__))));
-    $this->formCache = $this->getMockBuilder('Drupal\Core\Form\FormCache')
-      ->setConstructorArgs([$root, $this->keyValueExpirableFactory, $this->moduleHandler, $this->account, $this->csrfToken, $this->logger, $this->configFactory, $this->requestStack, $this->requestPolicy])
-      ->setMethods(['isPageCacheable'])
-      ->getMock();
-
-    $this->formCache->expects($this->once())
-      ->method('isPageCacheable')
-      ->willReturn(TRUE);
-
-    $this->formCache->setCache($form_build_id, $form, $form_state);
-  }
-
-  /**
    * Ensures SafeMarkup does not bleed from one test to another.
    */
   protected function resetSafeMarkup() {