Refactor EngineService to be able to inject custom Engine

fixes #38

test/service/engine_service_test.dart

@@ -1,19 +1,41 @@
 @TestOn('browser')
-
 import 'package:test/test.dart';
+import 'package:mockito/mockito.dart';
+import 'package:rules_of_living/src/Engine.dart';
 import 'package:rules_of_living/service/engine_service.dart';
 
+class MockEngine extends Mock implements Engine {}
+
 void main() {
   EngineService sut;
+  MockEngine me;
   setUp(() {
+    me = MockEngine();
     sut = EngineService();
   });
+  group("Dependency Injection", () {
+    test("EngineService accesses the Engine defined in getEngine", () {
+      sut.getEngine(me);
 
-  test("EngineService creates an engine on demand", () {
-    expect(sut.engine, isNotNull);
+      Engine result = sut.engine;
+      expect(result, equals(me));
+    });
   });
+  group("caching", () {
+    test("EngineService creates an engine on demand", () {
+      Engine result = sut.engine;
+      expect(result, TypeMatcher<Engine>());
+    });
 
-  test("EngineService returns the cached engine on subsequent requests", () {
-    expect(sut.engine, allOf(isNotNull, equals(sut.engine)));
+    test("EngineService returns the cached engine on subsequent requests", () {
+      Engine result = sut.engine;
+      expect(sut.engine, equals(result));
+    });
+    test("caching can be overriden by providing a custom engine", () {
+      Engine first = sut.engine;
+      sut.getEngine(me);
+      Engine second = sut.engine;
+      expect(second, isNot(equals(first)));
+    });
   });
 }