MSI: Update Magento 2 core to support MSI

Author: RomaKis

app/code/Magento/Store/Setup/InstallSchema.php

@@ -10,6 +10,7 @@
 use Magento\Framework\Setup\InstallSchemaInterface;
 use Magento\Framework\Setup\ModuleContextInterface;
 use Magento\Framework\Setup\SchemaSetupInterface;
+use Magento\Store\Api\Data\WebsiteInterface;
 
 /**
  * Recurring setup for Store module.
@@ -60,7 +61,7 @@ public function install(SchemaSetupInterface $setup, ModuleContextInterface $con
                 $installer->getTable('store_website'),
                 [
                     'website_id' => 0,
-                    'code' => 'admin',
+                    'code' => WebsiteInterface::ADMIN_CODE,
                     'name' => 'Admin',
                     'sort_order' => 0,
                     'default_group_id' => 0,