Sky: Allow clients to specify tile mode for gradients (repeating or mirror).

sky/examples/raw/painting.sky

Index: sky/examples/raw/painting.sky
diff --git a/sky/examples/raw/painting.sky b/sky/examples/raw/painting.sky
index 76da471c67e4b6058521110f9f6952d911f88feb..56a7bb9365bb338d930a0ab18745416d8f554bbd 100644
--- a/sky/examples/raw/painting.sky
+++ b/sky/examples/raw/painting.sky
@@ -68,9 +68,9 @@ void main() {
               ..setPaintBits(-1),
             (Paint layerPaint) {
           Gradient redYellow = new Gradient.Radial(
-              new Point(0.0, 0.0), radius,
+              new Point(0.0, 0.0), radius/3.0,
               [const Color(0xFFFFFF00), const Color(0xFFFF0000)],
-              null);
+              null, TileMode.mirror);
           layerPaint.setShader(redYellow);
         })
         ..addLayerOnTop(