Use an enum for Dart bindings for Skia Paint style

sky/engine/core/painting/PaintingStyle.dart

Index: sky/engine/core/painting/PaintingStyle.dart
diff --git a/sky/engine/core/painting/PaintingStyle.dart b/sky/engine/core/painting/PaintingStyle.dart
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+// 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.
+
+part of dart.sky;
+
+/// List of predefined painting styles. This list comes from Skia's
+/// SkPaint.h and the values (order) should be kept in sync.
+enum PaintingStyle {
+  fill,
+  stroke,
+  strokeAndFill,
+}