Add TransferMode to dart:sky, and use it instead of passing ints around.

sky/engine/core/painting/ColorFilter.h

Index: sky/engine/core/painting/ColorFilter.h
diff --git a/sky/engine/core/painting/ColorFilter.h b/sky/engine/core/painting/ColorFilter.h
index c62c4e1118a934128f3f9361d3fb2391ed0be2d3..5b879ee027eb13af7b8b6c28c5db772663657313 100644
--- a/sky/engine/core/painting/ColorFilter.h
+++ b/sky/engine/core/painting/ColorFilter.h
@@ -5,6 +5,8 @@
 #ifndef SKY_ENGINE_CORE_PAINTING_COLORFILTER_H_
 #define SKY_ENGINE_CORE_PAINTING_COLORFILTER_H_
 
+#include "sky/engine/core/painting/CanvasColor.h"
+#include "sky/engine/core/painting/TransferMode.h"
 #include "sky/engine/tonic/dart_wrappable.h"
 #include "sky/engine/wtf/PassRefPtr.h"
 #include "sky/engine/wtf/RefCounted.h"
@@ -16,7 +18,8 @@ class ColorFilter : public RefCounted<ColorFilter>, public DartWrappable {
   DEFINE_WRAPPERTYPEINFO();
  public:
   ~ColorFilter() override;
-  static PassRefPtr<ColorFilter> create(unsigned color, unsigned transfer_mode);
+  static PassRefPtr<ColorFilter> create(CanvasColor color,
+                                        TransferMode transfer_mode);
 
   SkColorFilter* filter() { return filter_.get(); }