[Site Editor]: Set html block as freeform fallback block (#48129)

packages/edit-site/src/index.js

@@ -4,6 +4,7 @@
 import { store as blocksStore } from '@wordpress/blocks';
 import {
 	registerCoreBlocks,
+	__experimentalGetCoreBlocks,
 	__experimentalRegisterExperimentalCoreBlocks,
 } from '@wordpress/block-library';
 import { dispatch } from '@wordpress/data';
@@ -16,7 +17,6 @@ import {
 import { store as editorStore } from '@wordpress/editor';
 import { store as interfaceStore } from '@wordpress/interface';
 import { store as preferencesStore } from '@wordpress/preferences';
-import { addFilter } from '@wordpress/hooks';
 import { registerLegacyWidgetBlock } from '@wordpress/widgets';
 
 /**
@@ -41,32 +41,17 @@ export function initializeEditor( id, settings ) {
 	settings.__experimentalFetchRichUrlData = fetchUrlData;
 
 	dispatch( blocksStore ).__experimentalReapplyBlockTypeFilters();
-	registerCoreBlocks();
+	const coreBlocks = __experimentalGetCoreBlocks().filter(
+		( { name } ) => name !== 'core/freeform'
+	);
+	registerCoreBlocks( coreBlocks );
+	dispatch( blocksStore ).setFreeformFallbackBlockName( 'core/html' );
 	registerLegacyWidgetBlock( { inserter: false } );
 	if ( process.env.IS_GUTENBERG_PLUGIN ) {
 		__experimentalRegisterExperimentalCoreBlocks( {
 			enableFSEBlocks: true,
 		} );
 	}
-	/*
-	 * Prevent adding the Clasic block in the site editor.
-	 * Only add the filter when the site editor is initialized, not imported.
-	 * Also only add the filter(s) after registerCoreBlocks()
-	 * so that common filters in the block library are not overwritten.
-	 *
-	 * This usage here is inspired by previous usage of the filter in the post editor:
-	 * https://github.com/WordPress/gutenberg/pull/37157
-	 */
-	addFilter(
-		'blockEditor.__unstableCanInsertBlockType',
-		'removeClassicBlockFromInserter',
-		( canInsert, blockType ) => {
-			if ( blockType.name === 'core/freeform' ) {
-				return false;
-			}
-			return canInsert;
-		}
-	);
 
 	// We dispatch actions and update the store synchronously before rendering
 	// so that we won't trigger unnecessary re-renders with useEffect.