make readonly dev-only

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

@@ -769,7 +769,9 @@ function serialize_inline_component(node, component_name, context) {
 
 			if (attribute.metadata.dynamic) {
 				// TODO create readonly deriveds outside the getters
-				push_prop(b.get(attribute.name, [b.return(b.call('$.readonly', value))]));
+				let arg = value;
+				if (context.state.options.dev) arg = b.call('$.readonly', arg);
+				push_prop(b.get(attribute.name, [b.return(arg)]));
 			} else {
 				push_prop(b.init(attribute.name, value));
 			}

packages/svelte/tests/runtime-runes/samples/proxy-prop-readonly/_config.js

@@ -3,6 +3,10 @@ import { test } from '../../test';
 export default test({
 	html: `<button>clicks: 0</button>`,
 
+	compileOptions: {
+		dev: true
+	},
+
 	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 		await btn?.click();