Error out on bad gridsize input

test/src/engine_test.dart

@@ -1,7 +1,7 @@
 import 'dart:html' as html;
+import 'dart:math';
 
 @TestOn('browser')
-
 import 'package:rules_of_living/src/Engine.dart';
 import 'package:test/test.dart';
 
@@ -31,5 +31,12 @@ void main() {
       expect(sut.canvas, isNotNull);
     });
   });
+  group("gridSize", () {
+    test("zero gridSizes throw ArgumentErrors", () {
+      expect(() => sut.gridSize = Point(0, 5), throwsArgumentError);
+    });
+    test("negative gridSizes throw ArgumentErrors", () {
+      expect(() => sut.gridSize = Point(1, -5), throwsArgumentError);
+    });
   });
 }