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

sky/engine/core/painting/Gradient.dart

Index: sky/engine/core/painting/Gradient.dart
diff --git a/sky/engine/core/painting/Gradient.dart b/sky/engine/core/painting/Gradient.dart
index 58381812fd72f42ef6709daa89458fcfab11d5eb..6eb199b8b340ef126b6fcde7eb710a8612cd23d3 100644
--- a/sky/engine/core/painting/Gradient.dart
+++ b/sky/engine/core/painting/Gradient.dart
@@ -4,26 +4,34 @@
 
 part of dart.sky;
 
+enum TileMode {
+  clamp,
+  repeated,
+  mirror
+}
+
 // Extends the generated _Gradient interface via the PrivateDart attribute.
 class Gradient extends _Gradient {
   // TODO(mpcomplete): Maybe pass a list of (color, colorStop) pairs instead?
   Gradient.Linear(List<Point> endPoints,
                   List<Color> colors,
-                  List<double> colorStops)
+                  List<double> colorStops,
+                  [TileMode tileMode = TileMode.clamp])
       : super() {
     if (endPoints == null || endPoints.length != 2)
       throw new ArgumentError("Expected exactly 2 [endPoints].");
     validateColorStops(colors, colorStops);
-    this._initLinear(endPoints, colors, colorStops);
+    this._initLinear(endPoints, colors, colorStops, tileMode.index);
   }
 
   Gradient.Radial(Point center,
                   double radius,
                   List<Color> colors,
-                  List<double> colorStops)
+                  List<double> colorStops,
+                  [TileMode tileMode = TileMode.clamp])
       : super() {
     validateColorStops(colors, colorStops);
-    this._initRadial(center, radius, colors, colorStops);
+    this._initRadial(center, radius, colors, colorStops, tileMode.index);
   }
 
   void validateColorStops(List<Color> colors, List<double> colorStops) {