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

Add a test for RenderDecoratedBox sets up the Paint object correctly.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1185423003.

examples/raw/shadow.dart

@@ -17,7 +17,7 @@ void beginFrame(double timeStamp) {
         new DrawLooperLayerInfo()
           ..setPaintBits(PaintBits.all)
           ..setOffset(const Point(5.0, 5.0))
-          ..setColorMode(TransferMode.srcIn),
+          ..setColorMode(TransferMode.src),
         (Paint layerPaint) {
       layerPaint.color = const Color.fromARGB(128, 55, 55, 55);
       layerPaint.setMaskFilter(

examples/rendering/shadowed_box.dart

@@ -16,7 +16,7 @@ void main() {
       gradient: new RadialGradient(
         center: Point.origin, radius: 500.0,
         colors: [Yellow[500], Blue[500]]),
-      boxShadow: Shadow[3])
+      boxShadow: shadows[3])
   );
   var paddedBox = new RenderPadding(
     padding: const EdgeDims.all(50.0),