Sky: Small fixes to Gradient interface. Added comments and renamed constructors.

sky/examples/raw/painting.sky

 <sky>
 <style>
 div {
     height: 200px;
     background-color: lightblue;
 }
 </style>
 <div id="canvas" />
 <script>
 import 'dart:math' as math;
 import 'dart:typed_data';
 import 'dart:sky';
 
 void main() {
   var element = document.getElementById('canvas');
   element.requestPaint((PaintingContext context) {
     Paint paint = new Paint();
     Point mid = new Point(context.width / 2.0, context.height / 2.0);
     double radius = math.min(mid.x, mid.y);
 
     context.save();
 
     context.clipRect(new Rect.fromLTRB(0.0, 0.0, context.width, radius));
 
     context.translate(mid.x, mid.y);
     paint.color = const Color.fromARGB(128, 255, 0, 255);
     context.rotate(math.PI/4.0);
 
-    Gradient yellowBlue = new Gradient.Linear(
+    Gradient yellowBlue = new Gradient.linear(
         [new Point(-radius, -radius), new Point(0.0, 0.0)],
-        [const Color(0xFFFFFF00), const Color(0xFF0000FF)],
-        null);
+        [const Color(0xFFFFFF00), const Color(0xFF0000FF)]);
     context.drawRect(new Rect.fromLTRB(-radius, -radius, radius, radius),
                      new Paint()..setShader(yellowBlue));
 
     // Scale x and y by 0.5.
     var scaleMatrix = new Float32List.fromList([
         0.5, 0.0, 0.0, 0.0,
         0.0, 0.5, 0.0, 0.0,
         0.0, 0.0, 0.0, 0.0,
         0.0, 0.0, 0.0, 1.0,
     ]);
@@ skipping 17 matching lines @@
                                    TransferMode.srcInMode));
           layerPaint.setMaskFilter(
               new MaskFilter.Blur(BlurStyle.normal, 3.0, highQuality: true));
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()
               ..setOffset(const Point(75.0, 75.0))
               ..setColorMode(TransferMode.srcMode)
               ..setPaintBits(-1),
             (Paint layerPaint) {
-          Gradient redYellow = new Gradient.Radial(
+          Gradient redYellow = new Gradient.radial(
               new Point(0.0, 0.0), radius/3.0,
               [const Color(0xFFFFFF00), const Color(0xFFFF0000)],
               null, TileMode.mirror);
           layerPaint.setShader(redYellow);
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()..setOffset(const Point(225.0, 75.0)),
             (Paint layerPaint) {
           // Since this layer uses a DST color mode, this has no effect.
           layerPaint.color = const Color.fromARGB(128, 255, 0, 0);
         });
     paint.setDrawLooper(builder.build());
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.commit();
   });
 }
 </script>
 </sky>