sapper/test/apps/preloading/src/routes/slow-preload.html

<h1>This page should never render</h1>

<script>
	export default {
		preload() {
			return new Promise(fulfil => {
				if (typeof window !== 'undefined') {
					window.fulfil = fulfil;
				} else {
					fulfil({});
				}
			});
		}
	};
</script>