return a Promise from goto - fixes #270

test/app/routes/about.html

@@ -6,15 +6,21 @@
 
 <p>This is the 'about' page. There's not much here.</p>
 
-<button class='goto' on:click='goto("blog/what-is-sapper")'>What is Sapper?</button>
 <button class='prefetch' on:click='prefetch("blog/why-the-name")'>Why the name?</button>
 
 <script>
 	import { goto, prefetch } from '../../../runtime.js';
 
 	export default {
+		oncreate() {
+			window.goto = goto;
+		},
+
+		ondestroy() {
+			window.goto = null;
+		},
+
 		methods: {
-			goto,
 			prefetch
 		}
 	};