docs(toh-6/dart): getHero makes HTTP get-by-id (#2936)

Dart follow-up to #2906

public/docs/_examples/toh-6/dart/lib/hero_service.dart

@@ -15,7 +15,7 @@ class HeroService {
   static final _headers = {'Content-Type': 'application/json'};
   // #enddocregion update
   // #docregion getHeroes
-  static const _heroesUrl = 'app/heroes'; // URL to web API
+  static const _heroesUrl = 'api/heroes'; // URL to web API
 
   final Client _http;
 
@@ -46,8 +46,16 @@ class HeroService {
   }
   // #enddocregion handleError, getHeroes
 
-  Future<Hero> getHero(int id) async =>
-      (await getHeroes()).firstWhere((hero) => hero.id == id);
+  // #docregion getHero
+  Future<Hero> getHero(int id) async {
+    try {
+      final response = await _http.get('$_heroesUrl/$id');
+      return new Hero.fromJson(_extractData(response));
+    } catch (e) {
+      throw _handleError(e);
+    }
+  }
+  // #enddocregion getHero
 
   // #docregion create
   Future<Hero> create(String name) async {
@@ -64,7 +72,7 @@ class HeroService {
 
   Future<Hero> update(Hero hero) async {
     try {
-      var url = '$_heroesUrl/${hero.id}';
+      final url = '$_heroesUrl/${hero.id}';
       final response =
           await _http.put(url, headers: _headers, body: JSON.encode(hero));
       return new Hero.fromJson(_extractData(response));
@@ -77,7 +85,7 @@ class HeroService {
   // #docregion delete
   Future<Null> delete(int id) async {
     try {
-      var url = '$_heroesUrl/$id';
+      final url = '$_heroesUrl/$id';
       await _http.delete(url, headers: _headers);
     } catch (e) {
       throw _handleError(e);

public/docs/_examples/toh-6/dart/lib/in_memory_data_service.dart

@@ -31,9 +31,14 @@ class InMemoryDataService extends MockClient {
     var data;
     switch (request.method) {
       case 'GET':
-        String prefix = request.url.queryParameters['name'] ?? '';
-        final regExp = new RegExp(prefix, caseSensitive: false);
-        data = _heroesDb.where((hero) => hero.name.contains(regExp)).toList();
+        final id = int.parse(request.url.pathSegments.last, onError: (_) => null);
+        if (id != null) {
+          data = _heroesDb.firstWhere((hero) => hero.id == id); // throws if no match
+        } else {
+          String prefix = request.url.queryParameters['name'] ?? '';
+          final regExp = new RegExp(prefix, caseSensitive: false);
+          data = _heroesDb.where((hero) => hero.name.contains(regExp)).toList();
+        }
         break;
       // #enddocregion init-disabled
       case 'POST':

public/docs/ts/_cache/tutorial/toh-pt6.jade

@@ -212,12 +212,32 @@ block get-heroes-details
   We've also decided to return a user friendly form of the error to
   the caller in a !{rejected_promise} so that the caller can display a proper error message to the user.
 
-  ### Unchanged `getHeroes` API
+  ### Get hero by id
+  The `HeroDetailComponent` asks the `HeroService` to fetch a single hero to edit.
   
-  Although we made significant *internal* changes to `getHeroes()`, the public signature did not change.
-  We still return a !{_Promise}. We won't have to update any of the components that call `getHeroes()`.
+  The `HeroService` currently fetches all heroes and then finds the desired hero 
+  by filtering for the one with the matching `id`.
+  That's fine in a simulation. It's wasteful to ask a real server for _all_ heroes when we only want one.
+  Most web APIs support a _get-by-id_ request in the form `api/hero/:id` (e.g., `api/hero/11`).
 
-  Our stakeholders are thrilled with the added flexibility from the API integration.
+  Update the `HeroService.getHero` method to make a _get-by-id_ request,
+  applying what we just learned to write `getHeroes`:
++makeExcerpt('app/hero.service.ts', 'getHero', '')
+:marked
+  It's almost the same as `getHeroes`. 
+  The URL identifies _which_ hero the server should update by encoding the hero id into the URL
+  to match the `api/hero/:id` pattern.
+
+  We also adjust to the fact that the `data` in the response is a single hero object rather than !{_an} !{_array}.
+
+  ### Unchanged _getHeroes_ API
+
+  Although we made significant *internal* changes to `getHeroes()` and `getHero()`, 
+  the public signatures did not change.
+  We still return a !{_Promise} from both methods. 
+  We won't have to update any of the components that call them.
+
+  Our stakeholders are thrilled with the web API integration so far.
   Now they want the ability to create and delete heroes.
 
   Let's see first what happens when we try to update a hero's details.
@@ -230,15 +250,12 @@ block get-heroes-details
   it. As we type, the hero name is updated in the view heading.
   But when we hit the `Back` button, the changes are lost!
 
-.l-sub-section
-  :marked
-    Updates weren't lost before, what's happening?
-    When the app used a list of mock heroes, changes were made directly to the
-    hero objects in the single, app-wide shared list. Now that we are fetching data
-    from a server, if we want changes to persist, we'll need to write them back to
-    the server.
+  Updates weren't lost before. What changed?
+  When the app used a list of mock heroes, updates were applied directly to the
+  hero objects within the single, app-wide, shared list. Now that we are fetching data
+  from a server, if we want changes to persist, we'll need to write them back to
+  the server.
 
-:marked
   ### Save hero details
 
   Let's ensure that edits to a hero's name aren't lost. Start by adding,

public/docs/ts/latest/tutorial/toh-pt6.jade

@@ -228,7 +228,7 @@ block get-heroes-details
   The URL identifies _which_ hero the server should update by encoding the hero id into the URL
   to match the `api/hero/:id` pattern.
 
-  We also adjust to the fact that the `data` in the response is a single hero object rather than array.
+  We also adjust to the fact that the `data` in the response is a single hero object rather than !{_an} !{_array}.
 
   ### Unchanged _getHeroes_ API