Expose and use constants for DrawLooperLayerInfo.setPaintBits to dart.

sky/engine/core/painting/DrawLooperLayerInfo.dart

+// 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;
+
+/// Paint masks for DrawLooperLayerInfo.setPaintBits. These specify which
+/// aspects of the layer's paint should replace the corresponding aspects on
+/// the draw's paint.
+///
+/// PaintBits.all means use the layer's paint completely.
+/// 0 means ignore the layer's paint... except for colorMode, which is
+/// always applied.
+class PaintBits {
+  static const int style       = 0x1;
+  static const int testSkewx   = 0x2;
+  static const int pathEffect  = 0x4;
+  static const int maskFilter  = 0x8;
+  static const int shader      = 0x10;
+  static const int colorFilter = 0x20;
+  static const int xfermode    = 0x40;
+  static const int all         = 0xFFFFFFFF;
+}