fix: don't blank css on migration error (#13703)

Author: paoloricciuti

.changeset/healthy-bees-wave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't blank css on migration error

packages/svelte/src/compiler/migrate/index.js

@@ -34,6 +34,7 @@ let has_migration_task = false;
  * @returns {{ code: string; }}
  */
 export function migrate(source, { filename } = {}) {
+	let og_source = source;
 	try {
 		has_migration_task = false;
 		// Blank CSS, could contain SCSS or similar that needs a preprocessor.
@@ -303,7 +304,7 @@ export function migrate(source, { filename } = {}) {
 		console.error('Error while migrating Svelte code', e);
 		has_migration_task = true;
 		return {
-			code: `<!-- @migration-task Error while migrating Svelte code: ${/** @type {any} */ (e).message} -->\n${source}`
+			code: `<!-- @migration-task Error while migrating Svelte code: ${/** @type {any} */ (e).message} -->\n${og_source}`
 		};
 	} finally {
 		if (has_migration_task) {

packages/svelte/tests/migrate/samples/not-blank-css-if-error/input.svelte

@@ -0,0 +1,11 @@
+<script>
+	export let error = true;
+</script>
+
+{$$props}
+
+<style lang="postcss">
+	div{
+		color: red;
+	}
+</style>

packages/svelte/tests/migrate/samples/not-blank-css-if-error/output.svelte

@@ -0,0 +1,12 @@
+<!-- @migration-task Error while migrating Svelte code: $$props is used together with named props in a way that cannot be automatically migrated. -->
+<script>
+	export let error = true;
+</script>
+
+{$$props}
+
+<style lang="postcss">
+	div{
+		color: red;
+	}
+</style>