Adds test case for drawAtlas in games

packages/flutter/example/game/test_drawatlas.dart

+import 'lib/sprites.dart';
+
+import 'dart:sky';
+
+import 'package:sky/mojo/asset_bundle.dart';
+import 'package:sky/rendering/object.dart';
+import 'package:sky/theme/colors.dart' as colors;
+import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/task_description.dart';
+import 'package:sky/widgets/theme.dart';
+
+AssetBundle _initBundle() {
+  if (rootBundle != null)
+    return rootBundle;
+  return new NetworkAssetBundle(Uri.base);
+}
+
+final AssetBundle _bundle = _initBundle();
+
+ImageMap _images;
+SpriteSheet _spriteSheet;
+TestDrawAtlasApp _app;
+
+main() async {
+  _images = new ImageMap(_bundle);
+
+  await _images.load([
+    'assets/sprites.png'
+  ]);
+
+  String json = await _bundle.loadString('assets/sprites.json');
+  _spriteSheet = new SpriteSheet(_images['assets/sprites.png'], json);
+
+  _app = new TestDrawAtlasApp();
+  runApp(_app);
+}
+
+class TestDrawAtlasApp extends App {
+
+  Widget build() {
+    ThemeData theme = new ThemeData(
+      brightness: ThemeBrightness.light,
+      primarySwatch: colors.Purple
+    );
+
+    return new Theme(
+      data: theme,
+      child: new TaskDescription(
+        label: 'Test drawAtlas',
+        child: new SpriteWidget(
+          new TestDrawAtlas(),
+          SpriteBoxTransformMode.fixedWidth
+        )
+      )
+    );
+  }
+}
+
+class TestDrawAtlas extends NodeWithSize {
+  TestDrawAtlas() : super(new Size(1024.0, 1024.0)) {
+  }
+
+  void paint(PaintingCanvas canvas) {
+    List<RSTransform> transforms = [
+      new RSTransform(1.0, 0.0, 100.0, 100.0)
+    ];
+    List<Rect> rects = [
+      _spriteSheet["ship.png"].frame
+    ];
+    List<Color> colors = [
+      new Color(0xffffffff)
+    ];
+
+    canvas.drawAtlas(
+      _spriteSheet.image,
+      transforms,
+      rects,
+      colors,
+      TransferMode.src,
+      null,
+      new Paint()..setFilterQuality(FilterQuality.low)..isAntiAlias=false
+    );
+  }
+}