fix: take async into consideration for dev delegated handlers (#17710)

Closes #17709

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- [x] It's really useful if your PR references an issue where it is
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reason. For large changes, please create an RFC:
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- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

.changeset/lemon-mails-guess.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: take async into consideration for dev delegated handlers

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/events.js

@@ -70,7 +70,8 @@ export function build_event(context, event_name, handler, capture, passive, dele
 		fn = b.function(
 			b.id(name),
 			handler.params,
-			handler.body.type === 'BlockStatement' ? handler.body : b.block([b.return(handler.body)])
+			handler.body.type === 'BlockStatement' ? handler.body : b.block([b.return(handler.body)]),
+			handler.async
 		);
 	}
 

packages/svelte/tests/runtime-runes/samples/event-handler-async-delegated/_config.js

@@ -0,0 +1,9 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	async test() {}
+});

packages/svelte/tests/runtime-runes/samples/event-handler-async-delegated/main.svelte

@@ -0,0 +1,8 @@
+<button
+	type="button"
+	onclick={async () => {
+		await Promise.resolve();
+	}}
+>
+	Button
+</button>