Support Svelte 3

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

test/apps/preloading/src/routes/preload-values/custom-class.html

@@ -1,17 +1,19 @@
-<h1>{foo.bar()}</h1>
+<script context="module">
+	export function preload() {
+		class Foo {
+			bar() {
+				return 42;
+			}
+		}
+
+		return {
+			foo: new Foo()
+		};
+	}
+</script>
 
 <script>
-	export default {
-		preload() {
-			class Foo {
-				bar() {
-					return 42;
-				}
-			}
+	export let foo;
+</script>
 
-			return {
-				foo: new Foo()
-			};
-		}
-	};
-</script>
+<h1>{foo.bar()}</h1>