[in_app_purchase] Update iOS to new analysis options (flutter#4920)

Author: stuartmorgan-g

packages/in_app_purchase/in_app_purchase_storekit/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.0+2
+
+* Internal code cleanup for stricter analysis options.
+
 ## 0.3.0+1
 
 * Removes dependency on `meta`.

packages/in_app_purchase/in_app_purchase_storekit/analysis_options.yaml

@@ -3,8 +3,8 @@
 // found in the LICENSE file.
 
 import 'package:flutter_test/flutter_test.dart';
-import 'package:in_app_purchase_storekit/in_app_purchase_storekit.dart';
 import 'package:in_app_purchase_platform_interface/in_app_purchase_platform_interface.dart';
+import 'package:in_app_purchase_storekit/in_app_purchase_storekit.dart';
 import 'package:integration_test/integration_test.dart';
 
 void main() {

packages/in_app_purchase/in_app_purchase_storekit/example/integration_test/in_app_purchase_test.dart

@@ -5,13 +5,14 @@
 import 'dart:async';
 import 'package:shared_preferences/shared_preferences.dart';
 
+// ignore: avoid_classes_with_only_static_members
 /// A store of consumable items.
 ///
-/// This is a development prototype tha stores consumables in the shared
+/// This is a development prototype that stores consumables in the shared
 /// preferences. Do not use this in real world apps.
 class ConsumableStore {
   static const String _kPrefKey = 'consumables';
-  static Future<void> _writes = Future.value();
+  static Future<void> _writes = Future<void>.value();
 
   /// Adds a consumable with ID `id` to the store.
   ///
@@ -32,19 +33,19 @@ class ConsumableStore {
   /// Returns the list of consumables from the store.
   static Future<List<String>> load() async {
     return (await SharedPreferences.getInstance()).getStringList(_kPrefKey) ??
-        [];
+        <String>[];
   }
 
   static Future<void> _doSave(String id) async {
-    List<String> cached = await load();
-    SharedPreferences prefs = await SharedPreferences.getInstance();
+    final List<String> cached = await load();
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
     cached.add(id);
     await prefs.setStringList(_kPrefKey, cached);
   }
 
   static Future<void> _doConsume(String id) async {
-    List<String> cached = await load();
-    SharedPreferences prefs = await SharedPreferences.getInstance();
+    final List<String> cached = await load();
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
     cached.remove(id);
     await prefs.setStringList(_kPrefKey, cached);
   }