Always run build:gutenberg -- --dev after downloading built asset

Gruntfile.js

@@ -1607,7 +1607,25 @@ module.exports = function(grunt) {
 			args: [ 'tools/gutenberg/download.js' ],
 			opts: { stdio: 'inherit' }
 		}, function( error ) {
-			done( ! error );
+			if ( error ) {
+				done( false );
+				return;
+			}
+			/*
+			 * Build block editor files into the src directory every time assets
+			 * are downloaded. This prevents failures when running from src
+			 * without running `build:dev` after those files were removed from
+			 * version control in https://core.trac.wordpress.org/changeset/61438.
+			 *
+			 * See https://core.trac.wordpress.org/ticket/64393.
+			 */
+			grunt.util.spawn( {
+				grunt: true,
+				args: [ 'build:gutenberg', '--dev' ],
+				opts: { stdio: 'inherit' }
+			}, function( buildError ) {
+				done( ! buildError );
+			} );
 		} );
 	} );
 

package.json

@@ -141,7 +141,7 @@
 		"typecheck:php": "node ./tools/local-env/scripts/docker.js run --rm php composer phpstan",
 		"gutenberg:copy": "node tools/gutenberg/copy.js",
 		"gutenberg:verify": "node tools/gutenberg/utils.js",
-		"gutenberg:download": "node tools/gutenberg/download.js",
+		"gutenberg:download": "node tools/gutenberg/download.js && grunt build:gutenberg --dev",
 		"vendor:copy": "node tools/vendors/copy-vendors.js",
 		"sync-gutenberg-packages": "grunt sync-gutenberg-packages",
 		"postsync-gutenberg-packages": "grunt wp-packages:sync-stable-blocks && grunt build --dev && grunt build"

tools/gutenberg/utils.js

@@ -42,13 +42,19 @@ function readGutenbergConfig() {
 }
 
 /**
- * Trigger a fresh download of the Gutenberg artifact by spawning download.js.
- * Exits the process if the download fails.
+ * Trigger a fresh download of the Gutenberg artifact by spawning download.js,
+ * then run `grunt build:gutenberg --dev` to copy the build to src/.
+ * Exits the process if either step fails.
  */
 function downloadGutenberg() {
-	const result = spawnSync( 'node', [ path.join( __dirname, 'download.js' ) ], { stdio: 'inherit' } );
-	if ( result.status !== 0 ) {
-		process.exit( result.status ?? 1 );
+	const downloadResult = spawnSync( 'node', [ path.join( __dirname, 'download.js' ) ], { stdio: 'inherit' } );
+	if ( downloadResult.status !== 0 ) {
+		process.exit( downloadResult.status ?? 1 );
+	}
+
+	const buildResult = spawnSync( 'grunt', [ 'build:gutenberg', '--dev' ], { stdio: 'inherit', shell: true } );
+	if ( buildResult.status !== 0 ) {
+		process.exit( buildResult.status ?? 1 );
 	}
 }