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

sky/engine/core/painting/TransferMode.h

+// 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.
+
+#ifndef SKY_ENGINE_CORE_PAINTING_TRANSFERMODE_H_
+#define SKY_ENGINE_CORE_PAINTING_TRANSFERMODE_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 "third_party/skia/include/core/SkXfermode.h"
+
+namespace blink {
+
+class TransferMode {
+ public:
+  SkXfermode::Mode sk_mode;
+};
+
+template<>
+struct DartConverter<TransferMode, void> {
+  static TransferMode FromArgumentsWithNullCheck(Dart_NativeArguments args,
+                                                 int index,
+                                                 Dart_Handle& exception);
+};
+
+} // namespace blink
+
+#endif  // SKY_ENGINE_CORE_PAINTING_TRANSFERMODE_H_