improve warning message

src/compiler/compile/render_dom/index.ts

@@ -413,7 +413,7 @@ export default function dom(
 				${unknown_props_check}
 
 				${component.slots.size || component.compile_options.dev ? b`let { $$slots = {}, $$scope } = $$props;` : null}
-				${component.compile_options.dev && b`@validate_slot($$slots, [${Array.from(component.slots.keys()).map(key => `"${key}"`).join(',')}]);`}
+				${component.compile_options.dev && b`@validate_slots('${component.tag}', $$slots, [${[...component.slots.keys()].map(key => `'${key}'`).join(',')}]);`}
 
 				${renderer.binding_groups.length > 0 && b`const $$binding_groups = [${renderer.binding_groups.map(_ => x`[]`)}];`}
 

src/runtime/internal/dev.ts

@@ -89,11 +89,10 @@ export function validate_each_argument(arg) {
 	}
 }
 
-export function validate_slot(slot, keys) {
-	keys = new Set(keys);
+export function validate_slots(name, slot, keys) {
 	for (const slot_key of Object.keys(slot)) {
-		if (!keys.has(slot_key)) {
-			console.warn(`Received unexpected slot named "${slot_key}"`);
+		if (!~keys.indexOf(slot_key)) {
+			console.warn(`<${name}> received an unexpected slot "${slot_key}".`);
 		}
 	}
 }