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

sky/engine/core/painting/TransferMode.cpp

+// 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.
+
+#include "sky/engine/config.h"
+#include "sky/engine/core/painting/TransferMode.h"
+
+#include "sky/engine/core/script/dom_dart_state.h"
+#include "sky/engine/tonic/dart_builtin.h"
+#include "sky/engine/tonic/dart_error.h"
+#include "sky/engine/tonic/dart_value.h"
+
+namespace blink {
+
+// Convert dart_mode => SkXfermode::Mode.
+TransferMode DartConverter<TransferMode, void>::FromArgumentsWithNullCheck(
+    Dart_NativeArguments args,
+    int index,
+    Dart_Handle& exception) {
+  TransferMode result;
+
+  Dart_Handle dart_mode = Dart_GetNativeArgument(args, index);
+  DCHECK(!LogIfError(dart_mode));
+
+  Dart_Handle value =
+      Dart_GetField(dart_mode, DOMDartState::Current()->value_handle());
+
+  uint64_t mode = 0;
+  Dart_Handle rv = Dart_IntegerToUint64(value, &mode);
+  DCHECK(!LogIfError(rv));
+
+  result.sk_mode = static_cast<SkXfermode::Mode>(mode);
+  return result;
+}
+
+} // namespace blink