Error out on bad gridsize input

lib/src/Engine.dart

@@ -32,7 +32,12 @@ class Engine {
   /// Grid Size
   ///
   /// Number of cells on x coordinate and y coordinate. Can be set individually.
-  Point gridSize = Point<int>(100, 100);
+  Point get gridSize => Point<int>(_grid.w, _grid.h);
+  void set gridSize(Point<int> value) {
+    if (value.x <= 0 || value.y <= 0)
+      throw ArgumentError("grid size must not be smaller than 1");
+    _grid = Grid(value.x, value.y);
+  }
 
   num _updateLag = 0.0;
   num _drawLag = 0.0;

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);
+    });
   });
 }