Add a simple inksplash example

sky/examples/raw/touch_demo.dart

Index: sky/examples/raw/touch_demo.dart
diff --git a/sky/examples/raw/touch_demo.dart b/sky/examples/raw/touch_demo.dart
index b90f86c8290f3441f80ff16e7b47b8c39036202e..1eac7fa103c18626164d88a2ee7a81a9796c9953 100644
--- a/sky/examples/raw/touch_demo.dart
+++ b/sky/examples/raw/touch_demo.dart
@@ -61,6 +61,10 @@ class RenderTouchDemo extends RenderBox {
     markNeedsPaint();
   }
 
+  void performLayout() {
+    size = constraints.constrain(new Size.infinite());
+  }
+
   void paint(RenderNodeDisplayList canvas) {
     dots.forEach((_, Dot dot) {
       dot.paint(canvas);