svelte/test/runtime/samples/dev-warning-readonly-window-binding/_config.js

export default {
	dev: true,

	test ( assert, component ) {
		try {
			component.set({ width: 99 });
			throw new Error( 'Expected an error' );
		} catch ( err ) {
			assert.equal( err.message, `Cannot set read-only property 'width'` );
		}
	}
};