diff --git a/core/lib/Drupal/Core/Extension/ModuleInstaller.php b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
index 0dfe322..6d605e8 100644
--- a/core/lib/Drupal/Core/Extension/ModuleInstaller.php
+++ b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
@@ -175,12 +175,13 @@ public function install(array $module_list, $enable_dependencies = TRUE) {
 
         // Update the module handler in order to load the module's code.
         // This allows the module to participate in hooks and its existence to
-        // be discovered by other modules.
+        // be discovered by other modules. The module code ("$module.module" and
+        // "$module.install" files) are loaded only after the kernel rebuild in
+        // order to make the PSR4 namespaces provided by this module available
+        // to its own code.
         // The current ModuleHandler instance is obsolete with the kernel
         // rebuild below.
         $this->moduleHandler->setModuleList($module_filenames);
-        $this->moduleHandler->load($module);
-        module_load_install($module);
 
         // Clear the static cache of system_rebuild_module_data() to pick up the
         // new module, since it merges the installation status of modules into
@@ -190,6 +191,10 @@ public function install(array $module_list, $enable_dependencies = TRUE) {
         // Update the kernel to include it.
         $this->updateKernel($module_filenames);
 
+        // Load the module code ("$module.module" and "$module.install" files).
+        $this->moduleHandler->load($module);
+        module_load_install($module);
+
         // Allow modules to react prior to the installation of a module.
         $this->moduleHandler->invokeAll('module_preinstall', array($module));
 
diff --git a/core/modules/aggregator/aggregator.module b/core/modules/aggregator/aggregator.module
index d4adae4..5180b75 100644
--- a/core/modules/aggregator/aggregator.module
+++ b/core/modules/aggregator/aggregator.module
@@ -6,6 +6,7 @@
  */
 
 use Drupal\aggregator\Entity\Feed;
+use Drupal\aggregator\FeedStorageInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 
 /**
@@ -14,7 +15,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\aggregator\FeedStorageInterface::CLEAR_NEVER instead.
  */
-const AGGREGATOR_CLEAR_NEVER = 0;
+const AGGREGATOR_CLEAR_NEVER = FeedStorageInterface::CLEAR_NEVER;
 
 /**
  * Implements hook_help().
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 0614d81..7ed1932 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -32,7 +32,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\comment\CommentInterface::ANONYMOUS_MAYNOT_CONTACT instead.
  */
-const COMMENT_ANONYMOUS_MAYNOT_CONTACT = 0;
+const COMMENT_ANONYMOUS_MAYNOT_CONTACT = CommentInterface::ANONYMOUS_MAYNOT_CONTACT;
 
 /**
  * Anonymous posters may leave their contact information.
@@ -40,7 +40,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\comment\CommentInterface::ANONYMOUS_MAY_CONTACT instead.
  */
-const COMMENT_ANONYMOUS_MAY_CONTACT = 1;
+const COMMENT_ANONYMOUS_MAY_CONTACT = CommentInterface::ANONYMOUS_MAY_CONTACT;
 
 /**
  * Anonymous posters are required to leave their contact information.
@@ -48,7 +48,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\comment\CommentInterface::ANONYMOUS_MUST_CONTACT instead.
  */
-const COMMENT_ANONYMOUS_MUST_CONTACT = 2;
+const COMMENT_ANONYMOUS_MUST_CONTACT = CommentInterface::ANONYMOUS_MUST_CONTACT;
 
 /**
  * The time cutoff for comments marked as read for entity types other node.
diff --git a/core/modules/menu_ui/menu_ui.module b/core/modules/menu_ui/menu_ui.module
index bbd8669..79cdcab 100644
--- a/core/modules/menu_ui/menu_ui.module
+++ b/core/modules/menu_ui/menu_ui.module
@@ -17,6 +17,7 @@
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\menu_link_content\Entity\MenuLinkContent;
+use Drupal\Core\Menu\MenuLinkManagerInterface;
 use Drupal\node\NodeTypeInterface;
 use Drupal\system\Entity\Menu;
 use Drupal\node\NodeInterface;
@@ -30,7 +31,7 @@
  *   Use \Drupal\Core\Menu\MenuLinkManagerInterface::MAX_MENU_NAME_LENGTH_UI
  *   instead.
  */
-const MENU_MAX_MENU_NAME_LENGTH_UI = 27;
+const MENU_MAX_MENU_NAME_LENGTH_UI = MenuLinkManagerInterface::MAX_MENU_NAME_LENGTH_UI;
 
 /**
  * Implements hook_help().
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index fbd1fb4..620afd6 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -35,7 +35,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\node\NodeInterface::NOT_PUBLISHED instead.
  */
-const NODE_NOT_PUBLISHED = 0;
+const NODE_NOT_PUBLISHED = NodeInterface::NOT_PUBLISHED;
 
 /**
  * Denotes that the node is published.
@@ -43,7 +43,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\node\NodeInterface::PUBLISHED instead.
  */
-const NODE_PUBLISHED = 1;
+const NODE_PUBLISHED = NodeInterface::PUBLISHED;
 
 /**
  * Denotes that the node is not promoted to the front page.
@@ -51,7 +51,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\node\NodeInterface::NOT_PROMOTED instead.
  */
-const NODE_NOT_PROMOTED = 0;
+const NODE_NOT_PROMOTED = NodeInterface::NOT_PROMOTED;
 
 /**
  * Denotes that the node is promoted to the front page.
@@ -59,7 +59,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\node\NodeInterface::PROMOTED instead.
  */
-const NODE_PROMOTED = 1;
+const NODE_PROMOTED = NodeInterface::PROMOTED;
 
 /**
  * Denotes that the node is not sticky at the top of the page.
@@ -67,7 +67,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\node\NodeInterface::NOT_STICKY instead.
  */
-const NODE_NOT_STICKY = 0;
+const NODE_NOT_STICKY = NodeInterface::NOT_STICKY;
 
 /**
  * Denotes that the node is sticky at the top of the page.
@@ -75,7 +75,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\node\NodeInterface::STICKY instead.
  */
-const NODE_STICKY = 1;
+const NODE_STICKY = NodeInterface::STICKY;
 
 /**
  * Implements hook_help().
diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index 4cd5cb1..4f88cad 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -11,6 +11,7 @@
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\image\Entity\ImageStyle;
 use Drupal\responsive_image\Entity\ResponsiveImageStyle;
+use Drupal\responsive_image\ResponsiveImageStyleInterface;
 use Drupal\Core\Image\ImageInterface;
 use Drupal\breakpoint\BreakpointInterface;
 
@@ -22,8 +23,8 @@
  *   and \Drupal\responsive_image\ResponsiveImageStyleInterface::ORIGINAL_IMAGE
  *   instead.
  */
-const RESPONSIVE_IMAGE_EMPTY_IMAGE = '_empty image_';
-const RESPONSIVE_IMAGE_ORIGINAL_IMAGE = '_original image_';
+const RESPONSIVE_IMAGE_EMPTY_IMAGE = ResponsiveImageStyleInterface::EMPTY_IMAGE;
+const RESPONSIVE_IMAGE_ORIGINAL_IMAGE = ResponsiveImageStyleInterface::ORIGINAL_IMAGE;
 
 /**
  * Implements hook_help().
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 1c77e23..13558c2 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -13,6 +13,7 @@
 use Drupal\Core\Database\Query\AlterableInterface;
 use Drupal\Core\Extension\Extension;
 use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Form\FormInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\KeyValueStore\KeyValueDatabaseExpirableFactory;
 use Drupal\Core\PageCache\RequestPolicyInterface;
@@ -58,7 +59,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\Core\Form\FormInterface::DISABLED instead.
  */
-const DRUPAL_DISABLED = 0;
+const DRUPAL_DISABLED = FormInterface::DISABLED;
 
 /**
  * Optional option on forms and settings.
@@ -66,7 +67,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\Core\Form\FormInterface::OPTIONAL instead.
  */
-const DRUPAL_OPTIONAL = 1;
+const DRUPAL_OPTIONAL = FormInterface::OPTIONAL;
 
 /**
  * Required option on forms and settings.
@@ -74,7 +75,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\Core\Form\FormInterface::REQUIRED instead.
  */
-const DRUPAL_REQUIRED = 2;
+const DRUPAL_REQUIRED = FormInterface::REQUIRED;
 
 /**
  * Return only visible regions.
diff --git a/core/modules/update/update.module b/core/modules/update/update.module
index f280e01..1a5711b 100644
--- a/core/modules/update/update.module
+++ b/core/modules/update/update.module
@@ -15,6 +15,8 @@
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Site\Settings;
+use Drupal\update\UpdateFetcherInterface;
+use Drupal\update\UpdateManagerInterface;
 
 /*
  * These are internally used constants for this code, do not modify.
@@ -26,7 +28,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateManagerInterface::NOT_SECURE instead.
  */
-const UPDATE_NOT_SECURE = 1;
+const UPDATE_NOT_SECURE = UpdateManagerInterface::NOT_SECURE;
 
 /**
  * Current release has been unpublished and is no longer available.
@@ -34,7 +36,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateManagerInterface::REVOKED instead.
  */
-const UPDATE_REVOKED = 2;
+const UPDATE_REVOKED = UpdateManagerInterface::REVOKED;
 
 /**
  * Current release is no longer supported by the project maintainer.
@@ -42,7 +44,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateManagerInterface::NOT_SUPPORTED instead.
  */
-const UPDATE_NOT_SUPPORTED = 3;
+const UPDATE_NOT_SUPPORTED = UpdateManagerInterface::NOT_SUPPORTED;
 
 /**
  * Project has a new release available, but it is not a security release.
@@ -50,7 +52,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateManagerInterface::NOT_CURRENT instead.
  */
-const UPDATE_NOT_CURRENT = 4;
+const UPDATE_NOT_CURRENT = UpdateManagerInterface::NOT_CURRENT;
 
 /**
  * Project is up to date.
@@ -58,7 +60,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateManagerInterface::CURRENT instead.
  */
-const UPDATE_CURRENT = 5;
+const UPDATE_CURRENT = UpdateManagerInterface::CURRENT;
 
 /**
  * Project's status cannot be checked.
@@ -66,7 +68,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateFetcherInterface::NOT_CHECKED instead.
  */
-const UPDATE_NOT_CHECKED = -1;
+const UPDATE_NOT_CHECKED = UpdateFetcherInterface::NOT_CHECKED;
 
 /**
  * No available update data was found for project.
@@ -74,7 +76,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateFetcherInterface::UNKNOWN instead.
  */
-const UPDATE_UNKNOWN = -2;
+const UPDATE_UNKNOWN = UpdateFetcherInterface::UNKNOWN;
 
 /**
  * There was a failure fetching available update data for this project.
@@ -82,7 +84,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateFetcherInterface::NOT_FETCHED instead.
  */
-const UPDATE_NOT_FETCHED = -3;
+const UPDATE_NOT_FETCHED = UpdateFetcherInterface::NOT_FETCHED;
 
 /**
  * We need to (re)fetch available update data for this project.
@@ -90,7 +92,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\update\UpdateFetcherInterface::FETCH_PENDING instead.
  */
-const UPDATE_FETCH_PENDING = -4;
+const UPDATE_FETCH_PENDING = UpdateFetcherInterface::FETCH_PENDING;
 
 /**
  * Implements hook_help().
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index 969ef89..26f7685 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -32,7 +32,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\user\UserInterface::USERNAME_MAX_LENGTH instead.
  */
-const USERNAME_MAX_LENGTH = 60;
+const USERNAME_MAX_LENGTH = UserInterface::USERNAME_MAX_LENGTH;
 
 /**
  * Only administrators can create user accounts.
@@ -40,7 +40,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\user\UserInterface::REGISTER_ADMINISTRATORS_ONLY instead.
  */
-const USER_REGISTER_ADMINISTRATORS_ONLY = 'admin_only';
+const USER_REGISTER_ADMINISTRATORS_ONLY = UserInterface::REGISTER_ADMINISTRATORS_ONLY;
 
 /**
  * Visitors can create their own accounts.
@@ -48,7 +48,7 @@
  * @deprecated in Drupal 8.3.x and will be removed before Drupal 9.0.x.
  *   Use \Drupal\user\UserInterface::REGISTER_VISITORS instead.
  */
-const USER_REGISTER_VISITORS = 'visitors';
+const USER_REGISTER_VISITORS = UserInterface::REGISTER_VISITORS;
 
 /**
  * Visitors can create accounts, but they don't become active without
@@ -58,7 +58,7 @@
  *   Use \Drupal\user\UserInterface::REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL
  * instead.
  */
-const USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL = 'visitors_admin_approval';
+const USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL = UserInterface::REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL;
 
 /**
  * Implements hook_help().