Support Svelte 3

fixes #546, #551, #552, #554

runtime/src/app/prefetchRoutes/index.ts

@@ -0,0 +1,13 @@
+import { components, routes } from '@sapper/internal/manifest-client';
+import { load_component } from '../app';
+
+export default function prefetchRoutes(pathnames: string[]) {
+	return routes
+		.filter(pathnames
+			? route => pathnames.some(pathname => route.pattern.test(pathname))
+			: () => true
+		)
+		.reduce((promise: Promise<any>, route) => promise.then(() => {
+			return Promise.all(route.parts.map(part => part && load_component(components[part.i])));
+		}), Promise.resolve());
+}