make canvas argument optional on instantiating engine

lib/src/Engine.dart

@@ -6,7 +6,7 @@ class Engine {
   // Elapsed Time Counter - useful for Safety Timeout
   Stopwatch _elapsed = new Stopwatch();
 
-  // Game Tick Rate - *does* impact game speed
+  // Game Tick Rate - *does* impact game speed TODO add configurable option
   int _MS_PER_STEP = 1000 ~/ 3;
 
   // Max Frame (i.e. Rendering) rate - does *not* impact game speed
@@ -15,14 +15,18 @@ class Engine {
   // ms stuck in updateloop after which game will declare itself unresponsive
   final int SAFETY_TIMEOUT = 2000;
 
+  // Grid Size TODO add as configurable option
+  static final GRID_X = 100;
+  static final GRID_Y = 100;
+
   num _updateLag = 0.0;
   num _drawLag = 0.0;
 
   final html.CanvasElement canvas;
-  Grid _grid = new Grid(100, 100);
+  Grid _grid = new Grid(GRID_X, GRID_Y);
   bool _running = false;
 
-  Engine(this.canvas) {
+  Engine([this.canvas]) {
     _elapsed.start();
     _grid.addPattern(amount: 15, dispersal: 5);
   }
@@ -59,7 +63,6 @@ class Engine {
   }
 
   void update() {
-//    print("updating");
     if (!_grid.update()) running = false;
   }
 

test/src/engine_test.dart

@@ -0,0 +1,12 @@
+@TestOn('browser')
+
+import 'dart:html';
+
+import 'package:rules_of_living/src/Engine.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("Engine can be instantiated without canvas", () {
+    expect(Engine(), isNot(throwsA(anything)));
+  });
+}