Allow setting a canvas for an engine at any point

test/src/engine_test.dart

@@ -1,22 +1,32 @@
+import 'dart:html' as html;
+
 @TestOn('browser')
 
 import 'package:rules_of_living/src/Engine.dart';
 import 'package:test/test.dart';
 
 void main() {
+  Engine sut;
+  setUp(() {
+    sut = Engine();
+  });
+
   test("Engine can be instantiated without canvas", () {
-    expect(Engine(), isNot(throwsNoSuchMethodError));
+    expect(sut, isNot(throwsNoSuchMethodError));
   });
 
   test("Engine does not throw errors when calling render directly", () {
-    Engine sut = new Engine();
     // anonymous function necessary since throws can not use functions with args
     expect(() => sut.render(), isNot(throwsNoSuchMethodError));
   });
 
   test("Engine does not throw errors when processing without attached canvas", () {
-    Engine sut = new Engine();
     // anonymous function necessary since throws can not use functions with args
     expect(() => sut.process(1000), isNot(throwsNoSuchMethodError));
   });
+
+  test("setCanvas allows setting a canvas for an engine at any point", () {
+    sut.canvas = new html.CanvasElement();
+    expect(sut.canvas, isNotNull);
+  });
 }