diff --git a/core/core.services.yml b/core/core.services.yml
index 2f7da56..d2aa505 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -569,6 +569,9 @@ services:
   entity.autocomplete_matcher:
     class: Drupal\Core\Entity\EntityAutocompleteMatcher
     arguments: ['@plugin.manager.entity_reference_selection']
+  block_form.manager:
+    class: Drupal\Core\Block\BlockFormManager
+    arguments: ['@class_resolver']
   plugin.manager.entity_reference_selection:
     class: Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManager
     parent: default_plugin_manager
diff --git a/core/modules/outside_in/src/Block/OutsideInBlockManager.php b/core/lib/Drupal/Core/Block/BlockFormManager.php
similarity index 91%
rename from core/modules/outside_in/src/Block/OutsideInBlockManager.php
rename to core/lib/Drupal/Core/Block/BlockFormManager.php
index 474f8a0..f866706 100644
--- a/core/modules/outside_in/src/Block/OutsideInBlockManager.php
+++ b/core/lib/Drupal/Core/Block/BlockFormManager.php
@@ -1,17 +1,17 @@
 <?php
 
-namespace Drupal\outside_in\Block;
+namespace Drupal\Core\Block;
 
 use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
 use Drupal\Component\Plugin\PluginInspectionInterface;
 use Drupal\Core\DependencyInjection\ClassResolverInterface;
 use Drupal\Core\Plugin\PluginFormInterface;
-use Drupal\outside_in\OperationAwareFormInterface;
+use Drupal\Core\Form\OperationAwareFormInterface;
 
 /**
  * Provides form discovery capabilities for block plugins.
  */
-class OutsideInBlockManager implements OutsideInBlockManagerInterface {
+class BlockFormManager implements BlockFormManagerInterface {
 
   /**
    * The class resolver.
@@ -21,7 +21,7 @@ class OutsideInBlockManager implements OutsideInBlockManagerInterface {
   protected $classResolver;
 
   /**
-   * OutsideInBlockManager constructor.
+   * BlockFormManager constructor.
    *
    * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
    */
diff --git a/core/modules/outside_in/src/Block/OutsideInBlockManagerInterface.php b/core/lib/Drupal/Core/Block/BlockFormManagerInterface.php
similarity index 89%
rename from core/modules/outside_in/src/Block/OutsideInBlockManagerInterface.php
rename to core/lib/Drupal/Core/Block/BlockFormManagerInterface.php
index 4fbed53..2135128 100644
--- a/core/modules/outside_in/src/Block/OutsideInBlockManagerInterface.php
+++ b/core/lib/Drupal/Core/Block/BlockFormManagerInterface.php
@@ -1,13 +1,13 @@
 <?php
 
-namespace Drupal\outside_in\Block;
+namespace Drupal\Core\Block;
 
 use Drupal\Component\Plugin\PluginInspectionInterface;
 
 /**
  * Provides form discovery capabilities for block plugins.
  */
-interface OutsideInBlockManagerInterface {
+interface BlockFormManagerInterface {
 
   /**
    * Creates a new form instance.
diff --git a/core/lib/Drupal/Core/Block/BlockManager.php b/core/lib/Drupal/Core/Block/BlockManager.php
index 3c88bdc..036933f 100644
--- a/core/lib/Drupal/Core/Block/BlockManager.php
+++ b/core/lib/Drupal/Core/Block/BlockManager.php
@@ -8,6 +8,7 @@
 use Drupal\Core\Plugin\CategorizingPluginManagerTrait;
 use Drupal\Core\Plugin\Context\ContextAwarePluginManagerTrait;
 use Drupal\Core\Plugin\DefaultPluginManager;
+use Drupal\Core\Plugin\PluginFormInterface;
 
 /**
  * Manages discovery and instantiation of block plugins.
@@ -48,6 +49,12 @@ public function __construct(\Traversable $namespaces, CacheBackendInterface $cac
   public function processDefinition(&$definition, $plugin_id) {
     parent::processDefinition($definition, $plugin_id);
     $this->processDefinitionCategory($definition);
+
+    // If no default form is defined and this plugin implements
+    // \Drupal\Core\Plugin\PluginFormInterface, use that for the default form.
+    if (!isset($definition['form']['default']) && is_subclass_of($definition['class'], PluginFormInterface::class)) {
+      $definition['form']['default'] = $definition['class'];
+    }
   }
 
   /**
diff --git a/core/modules/outside_in/src/OperationAwareFormInterface.php b/core/lib/Drupal/Core/Form/OperationAwareFormInterface.php
similarity index 91%
rename from core/modules/outside_in/src/OperationAwareFormInterface.php
rename to core/lib/Drupal/Core/Form/OperationAwareFormInterface.php
index cf31c58..8172bd7 100644
--- a/core/modules/outside_in/src/OperationAwareFormInterface.php
+++ b/core/lib/Drupal/Core/Form/OperationAwareFormInterface.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\outside_in;
+namespace Drupal\Core\Form;
 
 /**
  * Interface for forms that are aware of what operation they are performing.
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index eeec4d3..38272bd 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -3,6 +3,8 @@
 namespace Drupal\block;
 
 use Drupal\Component\Utility\Html;
+use Drupal\Core\Block\BlockFormManagerInterface;
+use Drupal\Core\Block\BlockPluginInterface;
 use Drupal\Core\Entity\EntityForm;
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Executable\ExecutableManagerInterface;
@@ -69,6 +71,13 @@ class BlockForm extends EntityForm {
   protected $contextRepository;
 
   /**
+   * The block form manager.
+   *
+   * @var \Drupal\Core\Block\BlockFormManagerInterface
+   */
+  protected $blockFormManager;
+
+  /**
    * Constructs a BlockForm object.
    *
    * @param \Drupal\Core\Entity\EntityManagerInterface $entity_manager
@@ -81,13 +90,16 @@ class BlockForm extends EntityForm {
    *   The language manager.
    * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
    *   The theme handler.
+   * @param \Drupal\Core\Block\BlockFormManagerInterface $block_form_manager
+   *   The block form manager.
    */
-  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, ContextRepositoryInterface $context_repository, LanguageManagerInterface $language, ThemeHandlerInterface $theme_handler) {
+  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, ContextRepositoryInterface $context_repository, LanguageManagerInterface $language, ThemeHandlerInterface $theme_handler, BlockFormManagerInterface $block_form_manager) {
     $this->storage = $entity_manager->getStorage('block');
     $this->manager = $manager;
     $this->contextRepository = $context_repository;
     $this->language = $language;
     $this->themeHandler = $theme_handler;
+    $this->blockFormManager = $block_form_manager;
   }
 
   /**
@@ -99,7 +111,8 @@ public static function create(ContainerInterface $container) {
       $container->get('plugin.manager.condition'),
       $container->get('context.repository'),
       $container->get('language_manager'),
-      $container->get('theme_handler')
+      $container->get('theme_handler'),
+      $container->get('block_form.manager')
     );
   }
 
@@ -120,7 +133,7 @@ public function form(array $form, FormStateInterface $form_state) {
     $form_state->setTemporaryValue('gathered_contexts', $this->contextRepository->getAvailableContexts());
 
     $form['#tree'] = TRUE;
-    $form['settings'] = $entity->getPlugin()->buildConfigurationForm(array(), $form_state);
+    $form['settings'] = $this->getPluginForm($this->entity->getPlugin())->buildConfigurationForm(array(), $form_state);
     $form['visibility'] = $this->buildVisibilityInterface([], $form_state);
 
     // If creating a new block, calculate a safe default machine name.
@@ -282,7 +295,7 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     // settings form element, so just pass that to the block for validation.
     $settings = (new FormState())->setValues($form_state->getValue('settings'));
     // Call the plugin validate handler.
-    $this->entity->getPlugin()->validateConfigurationForm($form, $settings);
+    $this->getPluginForm($this->entity->getPlugin())->validateConfigurationForm($form, $settings);
     // Update the original form values.
     $form_state->setValue('settings', $settings->getValues());
     $this->validateVisibility($form, $form_state);
@@ -322,15 +335,14 @@ protected function validateVisibility(array $form, FormStateInterface $form_stat
   public function submitForm(array &$form, FormStateInterface $form_state) {
     parent::submitForm($form, $form_state);
 
-    $entity = $this->entity;
     // The Block Entity form puts all block plugin form elements in the
     // settings form element, so just pass that to the block for submission.
     // @todo Find a way to avoid this manipulation.
     $settings = (new FormState())->setValues($form_state->getValue('settings'));
 
     // Call the plugin submit handler.
-    $entity->getPlugin()->submitConfigurationForm($form, $settings);
-    $block = $entity->getPlugin();
+    $block = $this->entity->getPlugin();
+    $this->getPluginForm($block)->submitConfigurationForm($form, $settings);
     // If this block is context-aware, set the context mapping.
     if ($block instanceof ContextAwarePluginInterface && $block->getContextDefinitions()) {
       $context_mapping = $settings->getValue('context_mapping', []);
@@ -339,7 +351,30 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
     // Update the original form values.
     $form_state->setValue('settings', $settings->getValues());
 
-    // Submit visibility condition settings.
+    $this->submitVisibility($form, $form_state);
+
+    // Save the settings of the plugin.
+    $this->entity->save();
+
+    drupal_set_message($this->t('The block configuration has been saved.'));
+    $form_state->setRedirect(
+      'block.admin_display_theme',
+      array(
+        'theme' => $form_state->getValue('theme'),
+      ),
+      array('query' => array('block-placement' => Html::getClass($this->entity->id())))
+    );
+  }
+
+  /**
+   * Helper function to independently submit the visibility UI.
+   *
+   * @param array $form
+   *   A nested array form elements comprising the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   */
+  protected function submitVisibility(array $form, FormStateInterface $form_state) {
     foreach ($form_state->getValue('visibility') as $condition_id => $values) {
       // Allow the condition to submit the form.
       $condition = $form_state->get(['conditions', $condition_id]);
@@ -354,20 +389,8 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       $condition_configuration = $condition->getConfiguration();
       $form_state->setValue(['visibility', $condition_id], $condition_configuration);
       // Update the visibility conditions on the block.
-      $entity->getVisibilityConditions()->addInstanceId($condition_id, $condition_configuration);
+      $this->entity->getVisibilityConditions()->addInstanceId($condition_id, $condition_configuration);
     }
-
-    // Save the settings of the plugin.
-    $entity->save();
-
-    drupal_set_message($this->t('The block configuration has been saved.'));
-    $form_state->setRedirect(
-      'block.admin_display_theme',
-      array(
-        'theme' => $form_state->getValue('theme'),
-      ),
-      array('query' => array('block-placement' => Html::getClass($this->entity->id())))
-    );
   }
 
   /**
@@ -402,4 +425,17 @@ public function getUniqueMachineName(BlockInterface $block) {
     return $machine_default;
   }
 
+  /**
+   * Retrieves the plugin form for a given block and operation.
+   *
+   * @param \Drupal\Core\Block\BlockPluginInterface $block
+   *   The block plugin.
+   *
+   * @return \Drupal\Core\Plugin\PluginFormInterface
+   *   The plugin form for the block.
+   */
+  protected function getPluginForm(BlockPluginInterface $block) {
+    return $this->blockFormManager->getFormObject($block, $this->operation);
+  }
+
 }
diff --git a/core/modules/outside_in/tests/modules/offcanvas_test/src/Form/OffCanvasForm.php b/core/modules/block/tests/modules/block_test/src/Form/SecondaryBlockForm.php
similarity index 81%
rename from core/modules/outside_in/tests/modules/offcanvas_test/src/Form/OffCanvasForm.php
rename to core/modules/block/tests/modules/block_test/src/Form/SecondaryBlockForm.php
index ecca616..3095963 100644
--- a/core/modules/outside_in/tests/modules/offcanvas_test/src/Form/OffCanvasForm.php
+++ b/core/modules/block/tests/modules/block_test/src/Form/SecondaryBlockForm.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\offcanvas_test\Form;
+namespace Drupal\block_test\Form;
 
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\OperationAwareFormInterface;
 use Drupal\Core\Plugin\PluginFormInterface;
-use Drupal\outside_in\OperationAwareFormInterface;
 
 /**
  * @todo.
  */
-class OffCanvasForm implements PluginFormInterface, OperationAwareFormInterface {
+class SecondaryBlockForm implements PluginFormInterface, OperationAwareFormInterface {
 
   /**
    * @var string
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
new file mode 100644
index 0000000..ff5ced3
--- /dev/null
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace Drupal\block_test\Plugin\Block;
+
+use Drupal\Core\Block\BlockBase;
+
+/**
+ * @todo.
+ *
+ * @Block(
+ *   id = "test_multiple_forms_block",
+ *   form = {
+ *     "secondary" = "\Drupal\block_test\Form\SecondaryBlockForm"
+ *   },
+ *   admin_label = @Translation("Multiple forms test block")
+ * )
+ */
+class TestMultipleFormsBlock extends BlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    return [];
+  }
+
+}
diff --git a/core/modules/block/tests/src/Unit/BlockFormTest.php b/core/modules/block/tests/src/Unit/BlockFormTest.php
index 09f450b..a27014a 100644
--- a/core/modules/block/tests/src/Unit/BlockFormTest.php
+++ b/core/modules/block/tests/src/Unit/BlockFormTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\block\Unit;
 
 use Drupal\block\BlockForm;
+use Drupal\Core\Block\BlockFormManagerInterface;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -55,6 +56,13 @@ class BlockFormTest extends UnitTestCase {
   protected $contextRepository;
 
   /**
+   * The block form manager.
+   *
+   * @var \Drupal\Core\Block\BlockFormManagerInterface|\Prophecy\Prophecy\ProphecyInterface
+   */
+  protected $blockFormManager;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUp() {
@@ -71,6 +79,7 @@ protected function setUp() {
       ->method('getStorage')
       ->will($this->returnValue($this->storage));
 
+    $this->blockFormManager = $this->prophesize(BlockFormManagerInterface::class);
   }
 
   /**
@@ -99,7 +108,7 @@ public function testGetUniqueMachineName() {
       ->method('getQuery')
       ->will($this->returnValue($query));
 
-    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->contextRepository, $this->language, $this->themeHandler);
+    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->contextRepository, $this->language, $this->themeHandler, $this->blockFormManager->reveal());
 
     // Ensure that the block with just one other instance gets the next available
     // name suggestion.
diff --git a/core/modules/outside_in/css/outside_in.module.css b/core/modules/outside_in/css/outside_in.module.css
index a199174..2578325 100644
--- a/core/modules/outside_in/css/outside_in.module.css
+++ b/core/modules/outside_in/css/outside_in.module.css
@@ -3,7 +3,7 @@
  * Styling for Outside-In module.
  */
 
-/* style the offcanvas container */
+/* Style the offcanvas container */
 #offcanvas {
   box-sizing: border-box;
   position: fixed;
@@ -12,51 +12,51 @@
   display: inline-block;
   width: 25%;
   -webkit-transform: translateX(100%);
-     -moz-transform: translateX(100%);
-       -o-transform: translateX(100%);
-      -ms-transform: translateX(100%);
-          transform: translateX(100%);
+  -moz-transform: translateX(100%);
+  -o-transform: translateX(100%);
+  -ms-transform: translateX(100%);
+  transform: translateX(100%);
   /* 2s delay matches transition speed to transform speed */
   -webkit-transition: all 1s ease-in-out 2s;
-     -moz-transition: all 1s ease-in-out 2s;
-          transition: all 1s ease-in-out 2s;
+  -moz-transition: all 1s ease-in-out 2s;
+  transition: all 1s ease-in-out 2s;
   z-index: 1000;
 }
 [dir="rtl"] #offcanvas {
   text-align: right;
   -webkit-transform: translateX(-100%);
-     -moz-transform: translateX(-100%);
-       -o-transform: translateX(-100%);
-      -ms-transform: translateX(-100%);
-          transform: translateX(-100%);
+  -moz-transform: translateX(-100%);
+  -o-transform: translateX(-100%);
+  -ms-transform: translateX(-100%);
+  transform: translateX(-100%);
 }
 #main-canvas-wrapper.js-tray-open #offcanvas {
   -webkit-transform: translateX(0);
-     -moz-transform: translateX(0);
-       -o-transform: translateX(0);
-      -ms-transform: translateX(0);
-          transform: translateX(0);
+  -moz-transform: translateX(0);
+  -o-transform: translateX(0);
+  -ms-transform: translateX(0);
+  transform: translateX(0);
 }
 #main-canvas-wrapper #main-canvas {
   display: inline-block;
   width: 100%;
   -webkit-transition: all 1s ease-in-out;
-     -moz-transition: all 1s ease-in-out;
-          transition: all 1s ease-in-out;
+  -moz-transition: all 1s ease-in-out;
+  transition: all 1s ease-in-out;
 }
 #main-canvas-wrapper.js-tray-open #main-canvas {
   left: 0; /* LTR */
   width: 75%;
   -webkit-transition: all 1s ease-in-out;
-     -moz-transition: all 1s ease-in-out;
-          transition: all 1s ease-in-out;
+  -moz-transition: all 1s ease-in-out;
+  transition: all 1s ease-in-out;
 }
 [dir="rtl"] #main-canvas-wrapper.js-tray-open #main-canvas {
   right: 0;
 }
-/* button that closes the offcanvas tray */
+/* Button that closes the offcanvas tray */
 #offcanvas > button.offcanvasClose,
-#offcanvas > button.offcanvasClose:hover {  /* todo: add hover state */
+#offcanvas > button.offcanvasClose:hover {  /* @todo add hover state */
   position: absolute;
   right: 15px; /* LTR */
   top: 15px;
@@ -95,9 +95,8 @@
   background-color: rgba(0,0,0,0.1);
 }
 
-/* Highlight editable regions in edit mode
-@tod Clean up this css.
-*/
+/* Highlight editable regions in edit mode. */
+/* @todo Clean up this css. */
 #main-canvas.js-outside-in-edit-mode .outside-in-editable {
   outline: -webkit-focus-ring-color auto 2px;
   outline-color: -webkit-focus-ring-color;
diff --git a/core/modules/outside_in/css/outside_in.theme.css b/core/modules/outside_in/css/outside_in.theme.css
index b67f380..1887e35 100644
--- a/core/modules/outside_in/css/outside_in.theme.css
+++ b/core/modules/outside_in/css/outside_in.theme.css
@@ -3,7 +3,7 @@
  * Visual styling for Outside-In module.
  */
 
-/* style the offcanvas container */
+/* Style the offcanvas container */
 div#offcanvas {
   background: #fff;
   border-left: 1px solid #ddd; /* LTR */
@@ -13,4 +13,3 @@ div#offcanvas {
   border-right: 1px solid #ddd;
   box-shadow: 2px 3px 1px 1px rgba(0, 0, 0, 0.3333);
 }
-
diff --git a/core/modules/outside_in/js/outside_in.js b/core/modules/outside_in/js/outside_in.js
index 70684df..8fd0f42 100644
--- a/core/modules/outside_in/js/outside_in.js
+++ b/core/modules/outside_in/js/outside_in.js
@@ -75,7 +75,7 @@
       Drupal.ajax(element_settings);
     });
 
-    // Bind a listener to all  'Quick Edit' links for blocks
+    // Bind a listener to all 'Quick Edit' links for blocks
     // Click "Edit" button in toolbar to force Contextual Edit which starts
     // Outside In edit mode also.
     data.$el.find('.outside-inblock-configure a').click(function (e) {
diff --git a/core/modules/outside_in/outside_in.module b/core/modules/outside_in/outside_in.module
index bfff508..db122c5 100644
--- a/core/modules/outside_in/outside_in.module
+++ b/core/modules/outside_in/outside_in.module
@@ -71,19 +71,6 @@ function outside_in_page_bottom(array &$page_bottom) {
 }
 
 /**
- * Implements hook_block_alter().
- */
-function outside_in_block_alter(&$definitions) {
-  foreach ($definitions as &$definition) {
-    // If no default form is defined and this plugin implements
-    // \Drupal\Core\Plugin\PluginFormInterface, use that for the default form.
-    if (!isset($definition['form']['default']) && is_subclass_of($definition['class'], PluginFormInterface::class)) {
-      $definition['form']['default'] = $definition['class'];
-    }
-  }
-}
-
-/**
  * Implements hook_entity_type_build().
  */
 function outside_in_entity_type_build(array &$entity_types) {
diff --git a/core/modules/outside_in/outside_in.services.yml b/core/modules/outside_in/outside_in.services.yml
index a71843f..c240a18 100644
--- a/core/modules/outside_in/outside_in.services.yml
+++ b/core/modules/outside_in/outside_in.services.yml
@@ -4,9 +4,7 @@ services:
       arguments: ['@title_resolver', '@renderer']
       tags:
         - { name: render.main_content_renderer, format: drupal_offcanvas }
-  outside_in.block.manager:
-    class: Drupal\outside_in\Block\OutsideInBlockManager
-    arguments: ['@class_resolver']
+
   outside_in.info:
     class: Drupal\outside_in\PageInfo
     arguments: ['@current_route_match', '@theme.negotiator.admin_theme']
diff --git a/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php b/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
index 2c5ad58..0445f0e 100644
--- a/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
+++ b/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
@@ -2,77 +2,34 @@
 
 namespace Drupal\outside_in\Block;
 
-use Drupal\Core\Entity\EntityForm;
+use Drupal\block\BlockForm;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Plugin\Context\ContextRepositoryInterface;
-use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
  * @todo.
  */
-class BlockEntityOffCanvasForm extends EntityForm {
+class BlockEntityOffCanvasForm extends BlockForm {
 
   /**
-   * The block entity.
-   *
-   * @var \Drupal\block\BlockInterface
-   */
-  protected $entity;
-
-  /**
-   * The block manager.
-   *
-   * @var \Drupal\outside_in\Block\OutsideInBlockManagerInterface
-   */
-  protected $blockManager;
-
-  /**
-   * The context repository service.
-   *
-   * @var \Drupal\Core\Plugin\Context\ContextRepositoryInterface
-   */
-  protected $contextRepository;
-
-  /**
-   * BlockEntityOffCanvasForm constructor.
-   *
-   * @param \Drupal\outside_in\Block\OutsideInBlockManagerInterface $block_manager
-   *   The block manager.
-   * @param \Drupal\Core\Plugin\Context\ContextRepositoryInterface $context_repository
-   *   The lazy context repository service.
+   * {@inheritdoc}
    */
-  public function __construct(OutsideInBlockManagerInterface $block_manager, ContextRepositoryInterface $context_repository) {
-    $this->contextRepository = $context_repository;
-    $this->blockManager = $block_manager;
+  protected function buildVisibilityInterface(array $form, FormStateInterface $form_state) {
+    // Do not display the visibility.
+    return [];
   }
 
   /**
    * {@inheritdoc}
    */
-  public static function create(ContainerInterface $container) {
-    return new static(
-      $container->get('outside_in.block.manager'),
-      $container->get('context.repository')
-    );
+  protected function validateVisibility(array $form, FormStateInterface $form_state) {
+    // Intentionally empty.
   }
 
   /**
    * {@inheritdoc}
    */
-  public function form(array $form, FormStateInterface $form_state) {
-    // Store theme settings in $form_state for use below.
-    $form_state->set('block_theme', $this->entity->getTheme());
-
-    // Store the gathered contexts in the form state for other objects to use
-    // during form building.
-    $form_state->setTemporaryValue('gathered_contexts', $this->contextRepository->getAvailableContexts());
-
-    $form['#tree'] = TRUE;
-    $form['#attached']['library'][] = 'block/drupal.block.admin';
-
-    $form['settings'] = $this->blockManager->getFormObject($this->entity->getPlugin(), 'offcavnas')->buildConfigurationForm([], $form_state);
-
-    return $form;
+  protected function submitVisibility(array $form, FormStateInterface $form_state) {
+    // Intentionally empty.
   }
 
 }
diff --git a/core/modules/outside_in/src/PageInfo.php b/core/modules/outside_in/src/PageInfo.php
index 22472d8..649439a 100644
--- a/core/modules/outside_in/src/PageInfo.php
+++ b/core/modules/outside_in/src/PageInfo.php
@@ -10,7 +10,6 @@
  */
 class PageInfo {
 
-
   /**
    * The admin theme negotiator.
    *
diff --git a/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php b/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php
index 2866311..da72f9b 100644
--- a/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php
+++ b/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php
@@ -10,9 +10,6 @@
  *
  * @Block(
  *   id = "offcanvas_links_block",
- *   form = {
- *     "offcanvas" = "\Drupal\offcanvas_test\Form\OffCanvasForm"
- *   },
  *   admin_label = @Translation("Off-canvas test block")
  * )
  */
diff --git a/core/modules/outside_in/tests/src/Kernel/MultipleBlockFormTest.php b/core/modules/outside_in/tests/src/Kernel/MultipleBlockFormTest.php
deleted file mode 100644
index 0899585..0000000
--- a/core/modules/outside_in/tests/src/Kernel/MultipleBlockFormTest.php
+++ /dev/null
@@ -1,37 +0,0 @@
-<?php
-
-namespace Drupal\Tests\outside_in\Kernel;
-
-use Drupal\KernelTests\KernelTestBase;
-use Drupal\offcanvas_test\Form\OffCanvasForm;
-
-/**
- * Tests that blocks can have multiple forms.
- *
- * @group outside_in
- */
-class MultipleBlockFormTest extends KernelTestBase {
-
-  /**
-   * @var array
-   */
-  public static $modules = ['system', 'block', 'outside_in', 'offcanvas_test'];
-
-  /**
-   * Tests that blocks can have multiple forms.
-   */
-  public function testMultipleForms() {
-    $block = \Drupal::service('plugin.manager.block')->createInstance('offcanvas_links_block');
-
-    $form_object1 = \Drupal::service('outside_in.block.manager')->getFormObject($block, 'default');
-    $form_object2 = \Drupal::service('outside_in.block.manager')->getFormObject($block, 'offcanvas');
-
-    // Assert that the block itself is used for the default form.
-    $this->assertSame($block, $form_object1);
-
-    $expected_offcanvas = new OffCanvasForm();
-    $expected_offcanvas->setOperation('offcanvas');
-    $this->assertEquals($expected_offcanvas, $form_object2);
-  }
-
-}
diff --git a/core/tests/Drupal/KernelTests/Core/Block/MultipleBlockFormTest.php b/core/tests/Drupal/KernelTests/Core/Block/MultipleBlockFormTest.php
new file mode 100644
index 0000000..ad13628
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Block/MultipleBlockFormTest.php
@@ -0,0 +1,37 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Block;
+
+use Drupal\block_test\Form\SecondaryBlockForm;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests that blocks can have multiple forms.
+ *
+ * @group block
+ */
+class MultipleBlockFormTest extends KernelTestBase {
+
+  /**
+   * @var array
+   */
+  public static $modules = ['system', 'block', 'block_test'];
+
+  /**
+   * Tests that blocks can have multiple forms.
+   */
+  public function testMultipleForms() {
+    $block = \Drupal::service('plugin.manager.block')->createInstance('test_multiple_forms_block');
+
+    $form_object1 = \Drupal::service('block_form.manager')->getFormObject($block, 'default');
+    $form_object2 = \Drupal::service('block_form.manager')->getFormObject($block, 'secondary');
+
+    // Assert that the block itself is used for the default form.
+    $this->assertSame($block, $form_object1);
+
+    $expected_secondary = new SecondaryBlockForm();
+    $expected_secondary->setOperation('secondary');
+    $this->assertEquals($expected_secondary, $form_object2);
+  }
+
+}
diff --git a/core/modules/outside_in/tests/src/Unit/OutsideInBlockManagerTest.php b/core/tests/Drupal/Tests/Core/Block/BlockFormManagerTest.php
similarity index 91%
rename from core/modules/outside_in/tests/src/Unit/OutsideInBlockManagerTest.php
rename to core/tests/Drupal/Tests/Core/Block/BlockFormManagerTest.php
index 1158516..7ea0e06 100644
--- a/core/modules/outside_in/tests/src/Unit/OutsideInBlockManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Block/BlockFormManagerTest.php
@@ -1,21 +1,21 @@
 <?php
 
-namespace Drupal\Tests\outside_in\Unit;
+namespace Drupal\Tests\Core\Block;
 
 use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
 use Drupal\Component\Plugin\PluginInspectionInterface;
 use Drupal\Core\DependencyInjection\ClassResolverInterface;
 use Drupal\Core\Plugin\PluginFormInterface;
-use Drupal\outside_in\OperationAwareFormInterface;
-use Drupal\outside_in\Block\OutsideInBlockManager;
+use Drupal\Core\Form\OperationAwareFormInterface;
+use Drupal\Core\Block\BlockFormManager;
 use Drupal\Tests\UnitTestCase;
 use Prophecy\Argument;
 
 /**
- * @coversDefaultClass \Drupal\outside_in\Block\OutsideInBlockManager
- * @group outside_in
+ * @coversDefaultClass \Drupal\Core\Block\BlockFormManager
+ * @group block
  */
-class OutsideInBlockManagerTest extends UnitTestCase {
+class BlockFormManagerTest extends UnitTestCase {
 
   /**
    * The class resolver.
@@ -27,7 +27,7 @@ class OutsideInBlockManagerTest extends UnitTestCase {
   /**
    * The manager being tested.
    *
-   * @var \Drupal\outside_in\Block\OutsideInBlockManager
+   * @var \Drupal\Core\Block\BlockFormManager
    */
   protected $manager;
 
@@ -38,7 +38,7 @@ protected function setUp() {
     parent::setUp();
 
     $this->classResolver = $this->prophesize(ClassResolverInterface::class);
-    $this->manager = new OutsideInBlockManager($this->classResolver->reveal());
+    $this->manager = new BlockFormManager($this->classResolver->reveal());
   }
 
   /**