Allow setting transform on radial gradients. (#4916)

lib/ui/painting.dart

@@ -1921,23 +1921,30 @@ class Gradient extends Shader {
   /// If `center`, `radius`, `colors`, or `tileMode` are null, or if `colors` or
   /// `colorStops` contain null values, this constructor will throw a
   /// [NoSuchMethodError].
+  /// 
+  /// If `matrix4` is provided, the gradient fill will be transformed by the 
+  /// specified 4x4 matrix relative to the local coordinate system. `matrix4` must
+  /// be a column-major matrix packed into a list of 16 values.
   Gradient.radial(
     Offset center,
     double radius,
     List<Color> colors, [
     List<double> colorStops,
     TileMode tileMode = TileMode.clamp,
+    Float64List matrix4
   ]) : assert(_offsetIsValid(center)),
        assert(colors != null),
        assert(tileMode != null),
        super._() {
+    if (matrix4 != null && matrix4.length != 16)
+      throw new ArgumentError('"matrix4" must have 16 entries.');
     _validateColorStops(colors, colorStops);
     final Int32List colorsBuffer = _encodeColorList(colors);
     final Float32List colorStopsBuffer = colorStops == null ? null : new Float32List.fromList(colorStops);
     _constructor();
-    _initRadial(center.dx, center.dy, radius, colorsBuffer, colorStopsBuffer, tileMode.index);
+    _initRadial(center.dx, center.dy, radius, colorsBuffer, colorStopsBuffer, tileMode.index, matrix4);
   }
-  void _initRadial(double centerX, double centerY, double radius, Int32List colors, Float32List colorStops, int tileMode) native 'Gradient_initRadial';
+  void _initRadial(double centerX, double centerY, double radius, Int32List colors, Float32List colorStops, int tileMode, Float64List matrix4) native 'Gradient_initRadial';
 
   static void _validateColorStops(List<Color> colors, List<double> colorStops) {
     if (colorStops == null) {

lib/ui/painting/gradient.cc

@@ -59,17 +59,24 @@ void CanvasGradient::initRadial(double center_x,
                                 double radius,
                                 const tonic::Int32List& colors,
                                 const tonic::Float32List& color_stops,
-                                SkShader::TileMode tile_mode) {
+                                SkShader::TileMode tile_mode,
+                                const tonic::Float64List& matrix4) {
   FXL_DCHECK(colors.num_elements() == color_stops.num_elements() ||
              color_stops.data() == nullptr);
 
   static_assert(sizeof(SkColor) == sizeof(int32_t),
                 "SkColor doesn't use int32_t.");
 
+  SkMatrix sk_matrix;
+  bool has_matrix = matrix4.data() != nullptr;
+  if (has_matrix) {
+    sk_matrix = ToSkMatrix(matrix4);
+  }
+
   set_shader(SkGradientShader::MakeRadial(
       SkPoint::Make(center_x, center_y), radius,
       reinterpret_cast<const SkColor*>(colors.data()), color_stops.data(),
-      colors.num_elements(), tile_mode));
+      colors.num_elements(), tile_mode, 0, has_matrix ? &sk_matrix : nullptr));
 }
 
 CanvasGradient::CanvasGradient() : Shader(nullptr) {}

lib/ui/painting/gradient.h

@@ -5,9 +5,11 @@
 #ifndef FLUTTER_LIB_UI_PAINTING_GRADIENT_H_
 #define FLUTTER_LIB_UI_PAINTING_GRADIENT_H_
 
+#include "flutter/lib/ui/painting/matrix.h"
 #include "flutter/lib/ui/painting/shader.h"
 #include "lib/tonic/dart_wrappable.h"
 #include "lib/tonic/typed_data/float32_list.h"
+#include "lib/tonic/typed_data/float64_list.h"
 #include "lib/tonic/typed_data/int32_list.h"
 #include "third_party/skia/include/effects/SkGradientShader.h"
 
@@ -38,7 +40,8 @@ class CanvasGradient : public Shader {
                   double radius,
                   const tonic::Int32List& colors,
                   const tonic::Float32List& color_stops,
-                  SkShader::TileMode tile_mode);
+                  SkShader::TileMode tile_mode,
+                  const tonic::Float64List& matrix4);
 
   static void RegisterNatives(tonic::DartLibraryNatives* natives);