Add Simulation Saving and Loading Methods

lib/src/Simulation.dart

@@ -10,6 +10,8 @@ enum CellPattern { SpaceShip, Blinker }
 
 class Simulation {
   Grid<bool> map;
+  Grid<bool> _snapshot;
+
   RuleSet rules = GameOfLife();
 
   bool _dirty = true;
@@ -151,4 +153,7 @@ class Simulation {
     _renderEdges = on;
     _dirty = true;
   }
+
+  void saveSnapshot() => _snapshot = map;
+  Grid<bool> loadSnapshot() => map = _snapshot;
 }

test/simulation_test.dart

@@ -5,6 +5,8 @@ import 'package:test/test.dart';
 
 import 'package:rules_of_living/src/Simulation.dart';
 
+class MockGrid extends Mock implements Grid<bool> {}
+
 void main() {
   Simulation sut;
   setUp(() {
@@ -34,4 +36,13 @@ void main() {
       expect(sut.dirty, true);
     }, skip: "can not find a way to set dirty to true first yet");
   });
+  group("save&load", () {
+    test("saves the current map, can be loaded by loadSnapshot", () {
+      var snapshot = sut.map;
+      sut.saveSnapshot();
+      sut.map = MockGrid();
+
+      expect(sut.loadSnapshot(), equals(snapshot));
+    });
+  });
 }