Re-land "Add a Color class to dart:sky."

The previous patch broke the sky tests. I have updated the framework to use the new Color class.

R=jackson@google.com

Review URL: https://codereview.chromium.org/1159663003

examples/raw/ink_well.dart

@@ -33,7 +33,7 @@ class InkSplash {
 
   void paint(RenderNodeDisplayList canvas) {
     int opacity = (_kInitialOpacity * (1.0 - (radius.value / _kTargetSize))).floor();
-    _paint.color = opacity << 24;
+    _paint.color = new sky.Color(opacity << 24);
     canvas.drawCircle(position.x, position.y, radius.value, _paint);
   }
 }
@@ -57,7 +57,7 @@ class InkWell extends RenderBox {
 
   void paint(RenderNodeDisplayList canvas) {
     canvas.drawRect(new sky.Rect.fromLTRB(0.0, 0.0, size.width, size.height),
-                    new sky.Paint()..color = 0xFFCCCCCC);
+                    new sky.Paint()..color = const sky.Color(0xFFCCCCCC));
     for (InkSplash splash in _splashes)
       splash.paint(canvas);
   }

examples/raw/painting.sky

@@ -22,7 +22,7 @@ void main() {
     context.clipRect(new Rect.fromLTRB(0.0, 0.0, context.width, radius));
 
     context.translate(mid.x, mid.y);
-    paint.setARGB(128, 255, 0, 255);
+    paint.color = const Color.fromARGB(128, 255, 0, 255);
     context.rotateDegrees(45.0);
 
     context.drawRect(new Rect.fromLTRB(-radius, -radius, radius, radius),
@@ -35,7 +35,7 @@ void main() {
         0.0, 0.0, 1.0
     ];
     context.concat(scaleMatrix);
-    paint.setARGB(128, 0, 255, 0);
+    paint.color = const Color.fromARGB(128, 0, 255, 0);
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.restore();
@@ -46,19 +46,19 @@ void main() {
             new DrawLooperLayerInfo()
               ..setOffset(150.0, 0.0)..setPaintBits(-1)..setColorMode(1),
             (Paint layerPaint) {
-          layerPaint.setARGB(128, 255, 255, 0);
+          layerPaint.color = const Color.fromARGB(128, 255, 255, 0);
           layerPaint.setColorFilter(new ColorFilter(0x770000ff, 5));
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()..setOffset(75.0, 75.0)..setColorMode(1),
             (Paint layerPaint) {
-          layerPaint.setARGB(128, 255, 0, 0);
+          layerPaint.color = const Color.fromARGB(128, 255, 0, 0);
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()..setOffset(225.0, 75.0),
             (Paint layerPaint) {
           // Since this layer uses a DST color mode, this has no effect.
-          layerPaint.setARGB(128, 255, 0, 0);
+          layerPaint.color = const Color.fromARGB(128, 255, 0, 0);
         });
     paint.setDrawLooper(builder.build());
     context.drawCircle(0.0, 0.0, radius, paint);