update validation logic and messages

- handle falsy {#each} arg
- only mention spread if this is an iterable

src/runtime/internal/dev.ts

@@ -80,15 +80,12 @@ export function set_data_dev(text, data) {
 }
 
 export function validate_each_argument(arg) {
-	if (arg instanceof Set || arg instanceof Map) {
+	if (!arg || !('length' in arg)) {
-		throw new Error(
+		let msg = '{#each} only iterates over array-like objects.';
-			`Svelte does not allow Sets or Maps in an {#each} block. Use [...arg.values()] or [...arg.entries()] instead.`
+		if (typeof Symbol === 'function' && arg && Symbol.iterator in arg) {
-		);
+			msg += ' You can use a spread to convert this iterable into an array.';
-	} 
+		}
-	if (arg.length === undefined) {
+		throw new Error(msg);
-		throw new Error(
-			`Svelte needs an array-like value for the {#each} block. You can spread your iterable into an array instead, e.g. [...iterable]`
-		);
 	}
 }