Add support for linear gradients, implemented as skia shaders.

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.
+
+// Extends the generated _Gradient interface via the CustomDart attribute.
+
+class Gradient extends _Gradient {
+  // TODO(mpcomplete): Support other gradient types.
+  // TODO(mpcomplete): Maybe pass a list of (color, colorStop) pairs instead?
+  Gradient.Linear(List<Point> endPoints,
+                  List<Color> colors,
+                  List<double> colorStops)
+      : super(0, endPoints, colors, _validate(colorStops, colors));
+
+  // TODO(mpcomplete): Figure out a good way to validate arguments.
+  static List<double> _validate(colorStops, colors) {
+    if (colorStops != null && colors.length != colorStops.length) {
+      throw new ArgumentError(
+          "[colors] and [colorStops] parameters must be equal length.");
+    }
+    return colorStops;
+  }
+}