Adds basic sprite sheet support to sprites

sky/examples/game/lib/spritesheet.dart

+part of sprites;
+
+class SpriteSheet {
+
+  Image _image;
+  Map<String, Texture> _textures = new Map();
+
+  SpriteSheet(this._image, String jsonDefinition) {
+    assert(_image != null);
+    assert(jsonDefinition != null);
+
+    JsonDecoder decoder = new JsonDecoder();
+    Map file = decoder.convert(jsonDefinition);
+    assert(file != null);
+
+    List frames = file["frames"];
+
+    for (Map frameInfo in frames) {
+      String fileName = frameInfo["filename"];
+      Rect frame = _readJsonRect(frameInfo["frame"]);
+      bool rotated = frameInfo["rotated"];
+      bool trimmed = frameInfo["trimmed"];
+      Rect spriteSourceSize = _readJsonRect(frameInfo["spriteSourceSize"]);
+      Size sourceSize = _readJsonSize(frameInfo["sourceSize"]);
+      Point pivot = _readJsonPoint(frameInfo["pivot"]);
+
+      var texture = new Texture._fromSpriteFrame(_image, fileName, sourceSize, rotated, trimmed, frame,
+        spriteSourceSize, pivot);
+      _textures[fileName] = texture;
+    }
+  }
+
+  Rect _readJsonRect(Map data) {
+    num x = data["x"];

[eseidel, 2015/06/23 23:22:31]

Do you mean to use num instead of int or double?

+    num y = data["y"];
+    num w = data["w"];
+    num h = data["h"];
+
+    return new Rect.fromLTRB(x.toDouble(), y.toDouble(), (x + w).toDouble(), (y + h).toDouble());
+  }
+
+  Size _readJsonSize(Map data) {
+    num w = data["w"];
+    num h = data["h"];
+
+    return new Size(w.toDouble(), h.toDouble());
+  }
+
+  Point _readJsonPoint(Map data) {
+    num x = data["x"];
+    num y = data["y"];
+
+    return new Point(x.toDouble(), y.toDouble());
+  }
+
+  Image get image => _image;
+
+  Texture operator [](String fileName) => _textures[fileName];
+}