example(dart/toh-4,5): getHeroesSlowly() to return getHeroes()

Have `getHeroesSlowly()` delay and then return the value of
`getHeroes()`. This makes it easier for user’s performing the tutorial
to keep this slower method as they evolve toh-5 into toh-6.

Author: chalin

public/docs/_examples/toh-4/dart/lib/hero_service.dart

@@ -17,7 +17,7 @@ class HeroService {
   // See the "Take it slow" appendix
   // #docregion get-heroes-slowly
   Future<List<Hero>> getHeroesSlowly() {
-    return new Future.delayed(const Duration(seconds: 2), () => mockHeroes);
+    return new Future.delayed(const Duration(seconds: 2), getHeroes);
   }
   // #enddocregion get-heroes-slowly
   // #docregion

public/docs/_examples/toh-4/dart/lib/hero_service_1.dart

@@ -13,7 +13,7 @@ class HeroService {
   // #enddocregion getHeroes-stub, empty-class, final
   /*
   // #docregion getHeroes-stub
-  List<Hero> getHeroes() {}
+  List<Hero> getHeroes() {} // stub
   // #enddocregion getHeroes-stub
   */
   // #docregion final

public/docs/_examples/toh-5/dart/lib/hero_service.dart

@@ -10,10 +10,9 @@ import 'mock_heroes.dart';
 class HeroService {
   Future<List<Hero>> getHeroes() async => mockHeroes;
 
-  // See the "Take it slow" appendix
   Future<List<Hero>> getHeroesSlowly() {
     return new Future<List<Hero>>.delayed(
-        const Duration(seconds: 2), () => mockHeroes);
+        const Duration(seconds: 2), getHeroes);
   }
 
   // #docregion getHero