preserve async/generator-ness of hoisted function expressions (#3179)

src/compiler/compile/nodes/shared/Expression.ts

@@ -363,7 +363,7 @@ export default class Expression {
 					}
 
 					const fn = deindent`
-						function ${name}(${args.join(', ')}) ${body}
+						${node.async && 'async '}function${node.generator && '*'} ${name}(${args.join(', ')}) ${body}
 					`;
 
 					if (dependencies.size === 0 && contextual_dependencies.size === 0) {

test/runtime/samples/event-handler-async/_config.js

@@ -0,0 +1,5 @@
+export default {
+	html: `
+		<button>nothing</button>
+	`,
+};

test/runtime/samples/event-handler-async/main.svelte

@@ -0,0 +1 @@
+<button on:click={async () => { await null; }}>nothing</button>