Sky: Add a DartConverterEnum and use that for all our enum needs.

sky/engine/core/painting/PaintingStyle.h

Index: sky/engine/core/painting/PaintingStyle.h
diff --git a/sky/engine/core/painting/PaintingStyle.h b/sky/engine/core/painting/PaintingStyle.h
index c08378563317c35cf76ab6b70ffc402dfbf86acd..374ea659fd0b332bdd4dd559cfbb8b0b3f59703a 100644
--- a/sky/engine/core/painting/PaintingStyle.h
+++ b/sky/engine/core/painting/PaintingStyle.h
@@ -5,10 +5,7 @@
 #ifndef SKY_ENGINE_CORE_PAINTING_PAINTINGSTYLE_H_
 #define SKY_ENGINE_CORE_PAINTING_PAINTINGSTYLE_H_
 
-#include "sky/engine/core/painting/Rect.h"
-#include "sky/engine/tonic/dart_wrappable.h"
-#include "sky/engine/wtf/PassRefPtr.h"
-#include "sky/engine/wtf/RefCounted.h"
+#include "sky/engine/tonic/dart_converter.h"
 #include "third_party/skia/include/core/SkPaint.h"
 
 namespace blink {
@@ -16,11 +13,12 @@ namespace blink {
 class PaintingStyle {};
 
 template <>
-struct DartConverter<PaintingStyle> {
-  static SkPaint::Style FromArgumentsWithNullCheck(Dart_NativeArguments args,
-                                                   int index,
-                                                   Dart_Handle& exception);
-};
+struct DartConverter<PaintingStyle> : public DartConverterEnum<SkPaint::Style> {};
+
+// If this fails, it's because SkPaint::Style has changed. We need to change
+// PaintingStyle.dart to ensure the PaintingStyle enum is in sync with the C++
+// values.
+COMPILE_ASSERT(SkPaint::kStyleCount == 3, Need_to_update_PaintingStyle_dart);
 
 } // namespace blink