After WP 5.0 is released and the Gutenberg plugin's minimum-supported version is 5.0, we should remove all PHP code from the plugin. This includes, but isn't limited to:

• WP_Block_Type_Registry, WP_Block_Type
• WP_REST_Autosaves_Controller
• WP_REST_Block_Renderer_Controller, WP_REST_Blocks_Controller
• WP_REST_Post_Search_Handler, WP_REST_Search_Controller, WP_REST_Search_Handler.
• lib/rest-api.php

We need to keep this PHP code in Gutenberg in the interim, so it's still possible to install Gutenberg on a 4.9.x installation.