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

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 bb1b027601827e9fa1aeaba5d1e0a69fa16377ac..eaed84c857d651f7b4e10b864954bbbca36b8e57 100644
--- a/sky/engine/core/painting/Gradient.dart
+++ b/sky/engine/core/painting/Gradient.dart
@@ -4,19 +4,26 @@
 
 part of dart.sky;
 
+/// Defines what happens at the edge of the gradient.
 enum TileMode {
+  /// Edge is clamped to the final color.
   clamp,
+  /// Edge is repeated from first color to last.
   repeated,
+  /// Edge is mirrored from last color to first.
   mirror
 }
 
-// Extends the generated _Gradient interface via the PrivateDart attribute.
+/// Extends the generated _Gradient interface via the PrivateDart attribute.
 class Gradient extends _Gradient {
+  /// Creates a linear gradient from [endPoint[0]] to [endPoint[1]]. If
+  /// [colorStops] is provided, [colorStops[i]] is a number from 0 to 1 that
+  /// specifies where [color[i]] begins in the gradient.
   // TODO(mpcomplete): Maybe pass a list of (color, colorStop) pairs instead?
-  Gradient.Linear(List<Point> endPoints,
+  Gradient.linear(List<Point> endPoints,
                   List<Color> colors,
-                  List<double> colorStops,
-                  [TileMode tileMode = TileMode.clamp])
+                  [List<double> colorStops = null,
+                  TileMode tileMode = TileMode.clamp])
       : super() {
     if (endPoints == null || endPoints.length != 2)
       throw new ArgumentError("Expected exactly 2 [endPoints].");
@@ -24,11 +31,15 @@ class Gradient extends _Gradient {
     this._initLinear(endPoints, colors, colorStops, tileMode);
   }
 
-  Gradient.Radial(Point center,
+  /// Creates a radial gradient centered at [center] that ends at [radius]
+  /// distance from the center. If [colorStops] is provided, [colorStops[i]] is
+  /// a number from 0 to 1 that specifies where [color[i]] begins in the
+  /// gradient.
+  Gradient.radial(Point center,
                   double radius,
                   List<Color> colors,
-                  List<double> colorStops,
-                  [TileMode tileMode = TileMode.clamp])
+                  [List<double> colorStops = null,
+                  TileMode tileMode = TileMode.clamp])
       : super() {
     validateColorStops(colors, colorStops);
     this._initRadial(center, radius, colors, colorStops, tileMode);