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

sky/engine/core/painting/Gradient.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 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].");
     validateColorStops(colors, colorStops);
     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);
   }
 
   void validateColorStops(List<Color> colors, List<double> colorStops) {
     if (colorStops != null && colors.length != colorStops.length) {
       throw new ArgumentError(
           "[colors] and [colorStops] parameters must be equal length.");
     }
   }
 }