Add a Paint::toString() method to describe our Paint objects.

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 61e43a9cdcf60974c01a951273ae61fd189f10fa..93fc5f29d04db6daadc7b769438462d1cfa8125e 100644
--- a/sky/examples/raw/painting.sky
+++ b/sky/examples/raw/painting.sky
@@ -50,20 +50,20 @@ void main() {
         ..addLayerOnTop(
             new DrawLooperLayerInfo()
               ..setOffset(const Point(150.0, 0.0))
-              ..setColorMode(TransferMode.srcMode)
+              ..setColorMode(TransferMode.src)
               ..setPaintBits(PaintBits.all),
             (Paint layerPaint) {
           layerPaint.color = const Color.fromARGB(128, 255, 255, 0);
           layerPaint.setColorFilter(
               new ColorFilter.mode(const Color.fromARGB(128, 0, 0, 255),
-                                   TransferMode.srcInMode));
+                                   TransferMode.srcIn));
           layerPaint.setMaskFilter(
               new MaskFilter.blur(BlurStyle.normal, 3.0, highQuality: true));
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()
               ..setOffset(const Point(75.0, 75.0))
-              ..setColorMode(TransferMode.srcMode)
+              ..setColorMode(TransferMode.src)
               ..setPaintBits(PaintBits.shader),
             (Paint layerPaint) {
           Gradient redYellow = new Gradient.radial(