use experimental.async in sandbox and migrate

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

@@ -146,7 +146,10 @@ export function migrate(source, { filename, use_ts } = {}) {
 			...validate_component_options({}, ''),
 			...parsed_options,
 			customElementOptions,
-			filename: filename ?? '(unknown)'
+			filename: filename ?? '(unknown)',
+			experimental: {
+				async: true
+			}
 		};
 
 		const str = new MagicString(source);

playgrounds/sandbox/run.js

@@ -67,7 +67,10 @@ for (const generate of /** @type {const} */ (['client', 'server'])) {
 			dev: true,
 			filename: input,
 			generate,
-			runes: argv.values.runes
+			runes: argv.values.runes,
+			experimental: {
+				async: true
+			}
 		});
 
 		for (const warning of compiled.warnings) {
@@ -94,7 +97,10 @@ for (const generate of /** @type {const} */ (['client', 'server'])) {
 		const compiled = compileModule(source, {
 			dev: true,
 			filename: input,
-			generate
+			generate,
+			experimental: {
+				async: true
+			}
 		});
 
 		const output_js = `${cwd}/output/${generate}/${file}`;