fix: async `class:` + spread attributes were compiled into sync server-side code (#16834)

.changeset/tasty-snails-dress.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: async `class:` + spread attributes were compiled into sync server-side code

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js

@@ -372,7 +372,10 @@ function build_element_spread_attributes(
 				directive.name,
 				directive.expression.type === 'Identifier' && directive.expression.name === directive.name
 					? b.id(directive.name)
-					: /** @type {Expression} */ (context.visit(directive.expression))
+					: transform(
+							/** @type {Expression} */ (context.visit(directive.expression)),
+							directive.metadata.expression
+						)
 			);
 		});
 

packages/svelte/tests/server-side-rendering/samples/async-directive-with-spreading/_expected.html

@@ -0,0 +1 @@
+<!--[--><div class="test"></div><div style="color: green;"></div><!--]-->

packages/svelte/tests/server-side-rendering/samples/async-directive-with-spreading/main.svelte

@@ -0,0 +1,2 @@
+<div class:test={await true} {...{}}></div>
+<div style:color={await "green"} {...{}}></div>