Enable cached engine returns for EngineService

lib/service/engine_service.dart

@@ -5,13 +5,15 @@ import 'package:rules_of_living/src/Engine.dart';
 class EngineService {
   Engine _engine;
 
-  Engine get engine => _engine ?? create(null);
+  Engine get engine => _engine ?? createEngine(null);
 
-  Engine create(html.CanvasElement canvas) {
+  Engine createEngine(html.CanvasElement canvas) {
     _engine = Engine(canvas);
     return _engine;
   }
 
+  void setCanvas(html.CanvasElement canvas) => engine.canvas = canvas;
+
   void run() {
     engine.running = true;
   }

test/service/engine_service_test.dart

@@ -0,0 +1,19 @@
+@TestOn('browser')
+
+import 'package:test/test.dart';
+import 'package:rules_of_living/service/engine_service.dart';
+
+void main() {
+  EngineService sut;
+  setUp(() {
+    sut = EngineService();
+  });
+
+  test("EngineService creates an engine on demand", () {
+    expect(sut.engine, isNotNull);
+  });
+
+  test("EngineService returns the cached engine on subsequent requests", () {
+    expect(sut.engine, allOf(isNotNull, equals(sut.engine)));
+  });
+}