Adds basic sprite sheet support to sprites

sky/examples/game/lib/sprites.dart

Index: sky/examples/game/lib/sprites.dart
diff --git a/sky/examples/game/lib/sprites.dart b/sky/examples/game/lib/sprites.dart
index 3f31231d5c04b8159a2abf447569602df8502ab2..840e19f319d3badf348524b2ccdf7563b5c1507f 100644
--- a/sky/examples/game/lib/sprites.dart
+++ b/sky/examples/game/lib/sprites.dart
@@ -3,6 +3,7 @@ library sprites;
 import 'dart:math' as Math;
 import 'dart:sky';
 import 'dart:typed_data';
+import 'dart:convert';
 
 import 'package:sky/base/scheduler.dart' as scheduler;
 import 'package:sky/mojo/net/image_cache.dart' as image_cache;
@@ -10,6 +11,7 @@ import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:sky/widgets/widget.dart';
 import 'package:vector_math/vector_math.dart';
+import 'package:sky/framework/net/fetch.dart';
 
 part 'sprite_box.dart';
 part 'sprite_widget.dart';
@@ -17,3 +19,5 @@ part 'node.dart';
 part 'node_with_size.dart';
 part 'sprite.dart';
 part 'image_map.dart';
+part 'texture.dart';
+part 'spritesheet.dart';