Adds particle systems to sprites

sky/sdk/example/game/lib/sprite_box.dart

 part of sprites;
 
 /// Options for setting up a [SpriteBox].
 ///
 ///  * [nativePoints], use the same points as the parent [Widget].
 ///  * [letterbox], use the size of the root node for the coordinate system, constrain the aspect ratio and trim off
 ///  areas that end up outside the screen.
 ///  * [stretch], use the size of the root node for the coordinate system, scale it to fit the size of the box.
 ///  * [scaleToFit], similar to the letterbox option, but instead of trimming areas the sprite system will be scaled
 ///  down to fit the box.
@@ skipping 275 matching lines @@
 
     // Print frame rate
     if (_numFrames % 60 == 0) print("delta: $delta fps: $_frameRate");
 
     _callUpdate(_rootNode, delta);
     _scheduleTick();
   }
 
   void _callUpdate(Node node, double dt) {
     node.update(dt);
-    for (Node child in node.children) {
+    for (int i = node.children.length - 1; i >= 0; i--) {
+      Node child = node.children[i];
       if (!child.paused) {
         _callUpdate(child, dt);
       }
     }
   }
 
   void _callSpriteBoxPerformedLayout(Node node) {
     node.spriteBoxPerformedLayout();
     for (Node child in node.children) {
       _callSpriteBoxPerformedLayout(child);
@@ skipping 66 matching lines @@
   ///     if (event.pointer == firstPointerId) {
   ///       // Do something
   ///     }
   final int pointer;
 
   /// Creates a new SpriteBoxEvent, typically this is done internally inside the SpriteBox.
   ///
   ///     var event = new SpriteBoxEvent(new Point(50.0, 50.0), 'pointerdown', 0);
   SpriteBoxEvent(this.boxPosition, this.type, this.pointer);
 }