Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/CanvasColor.h

Index: sky/engine/core/painting/CanvasColor.h
diff --git a/sky/engine/core/painting/CanvasColor.h b/sky/engine/core/painting/CanvasColor.h
index 04205f6e4927ed29005a7dfba7ef31da35f90206..0e727ea0416bf552f06a662684a3f533ca894516 100644
--- a/sky/engine/core/painting/CanvasColor.h
+++ b/sky/engine/core/painting/CanvasColor.h
@@ -16,7 +16,14 @@ namespace blink {
 class CanvasColor {};
 
 template <>
+struct DartConverterTypes<CanvasColor> {
+  using ConverterType = CanvasColor;
+  using ValueType = SkColor;
+};
+
+template <>
 struct DartConverter<CanvasColor> {
+  static SkColor FromDart(Dart_Handle handle);
   static SkColor FromArgumentsWithNullCheck(Dart_NativeArguments args,
                                             int index,
                                             Dart_Handle& exception);