Change router examples to use SynchronousFuture (#796)

navigation_and_routing/lib/router_advanced/transition_delegate.dart

@@ -6,6 +6,7 @@
 /// transition animations are shown. (For example, [when two routes are popped
 /// off the stack](https://github.com/flutter/flutter/issues/12146), however the
 /// default TransitionDelegate will handle this if you are using Router)
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 
 void main() {
@@ -27,7 +28,7 @@ class BooksApp extends StatefulWidget {
 class _BooksAppState extends State<BooksApp> {
   BookRouterDelegate _routerDelegate = BookRouterDelegate();
   BookRouteInformationParser _routeInformationParser =
-  BookRouteInformationParser();
+      BookRouteInformationParser();
 
   @override
   Widget build(BuildContext context) {
@@ -42,14 +43,15 @@ class _BooksAppState extends State<BooksApp> {
 class BookRouteInformationParser extends RouteInformationParser<BookRoutePath> {
   @override
   Future<BookRoutePath> parseRouteInformation(
-      RouteInformation routeInformation) async {
+    RouteInformation routeInformation,
+  ) {
     final uri = Uri.parse(routeInformation.location);
 
     if (uri.pathSegments.length >= 2) {
       var remaining = uri.pathSegments[1];
-      return BookRoutePath.details(int.tryParse(remaining));
+      return SynchronousFuture(BookRoutePath.details(int.tryParse(remaining)));
     } else {
-      return BookRoutePath.home();
+      return SynchronousFuture(BookRoutePath.home());
     }
   }
 
@@ -113,10 +115,11 @@ class BookRouterDelegate extends RouterDelegate<BookRoutePath>
   }
 
   @override
-  Future<void> setNewRoutePath(BookRoutePath path) async {
+  Future<void> setNewRoutePath(BookRoutePath path) {
     if (path.isDetailsPage) {
       _selectedBook = books[path.id];
     }
+    return SynchronousFuture<void>(null);
   }
 
   void _handleBookTapped(Book book) {
@@ -213,9 +216,9 @@ class NoAnimationTransitionDelegate extends TransitionDelegate<void> {
   Iterable<RouteTransitionRecord> resolve({
     List<RouteTransitionRecord> newPageRouteHistory,
     Map<RouteTransitionRecord, RouteTransitionRecord>
-    locationToExitingPageRoute,
+        locationToExitingPageRoute,
     Map<RouteTransitionRecord, List<RouteTransitionRecord>>
-    pageRouteToPagelessRoutes,
+        pageRouteToPagelessRoutes,
   }) {
     final results = <RouteTransitionRecord>[];