throw error for unsupported browser in dev

src/runtime/internal/dev.ts

@@ -1,4 +1,4 @@
-import { custom_event, append, append_hydration, insert, insert_hydration, detach, listen, attr } from './dom';
+import { custom_event, add_resize_observer, append, append_hydration, insert, insert_hydration, detach, listen, attr } from './dom';
 import { SvelteComponent } from './Component';
 
 export function dispatch_dev<T=any>(type: string, detail?: T) {
@@ -89,6 +89,14 @@ export function set_data_dev(text, data) {
 	text.data = data;
 }
 
+export function add_resize_observer_dev(node: HTMLElement, fn: (arg: any) => void, binding_name: string) {
+	if ('ResizeObserver' in  window) {
+		add_resize_observer(node, fn);
+	} else {
+		throw new Error(`${binding_name} uses the ResizeObserver API, but it is not supported in this browser.`);
+	}
+}
+
 export function validate_each_argument(arg) {
 	if (typeof arg !== 'string' && !(arg && typeof arg === 'object' && 'length' in arg)) {
 		let msg = '{#each} only iterates over array-like objects.';

src/runtime/internal/dom.ts

@@ -631,14 +631,9 @@ export function add_resize_listener(node: HTMLElement, fn: () => void) {
 }
 
 export function add_resize_observer(node: HTMLElement, fn: (arg: any) => void) {
-	if ('ResizeObserver' in window) {
+	const observer = new ResizeObserver(entries => fn(entries.values().next().value));
-		// @ts-ignore https://github.com/Microsoft/TypeScript/issues/28502
+	observer.observe(node);
-		const observer = new ResizeObserver(entries => fn(entries.values().next().value));
+	return () => observer.disconnect();
-		observer.observe(node);
-		return () => observer.disconnect();
-	} else {
-		return noop;
-	}
 }
 
 export function toggle_class(element, name, toggle) {