Support Svelte 3

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

test/apps/css/src/routes/bar.html

@@ -1,11 +1,13 @@
-<svelte:component this={Title}/>
-
 <script>
-	export default {
-		oncreate() {
-			import('./_components/Title.html').then(({ default: Title }) => {
-				this.set({ Title });
-			});
-		}
-	};
-</script>
+	import { onMount } from 'svelte';
+
+	export let Title;
+
+	onMount(() => {
+		import('./_components/Title.html').then(mod => {
+			Title = mod.default;
+		});
+	});
+</script>
+
+<svelte:component this={Title}/>