Re-land "Add a Color class to dart:sky."

sky/engine/core/painting/Color.dart

Index: sky/engine/core/painting/Color.dart
diff --git a/sky/engine/core/painting/Color.dart b/sky/engine/core/painting/Color.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b88e3b45619e84bac46be5ead6773ac2e3a20fcc
--- /dev/null
+++ b/sky/engine/core/painting/Color.dart
@@ -0,0 +1,18 @@
+// 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;
+
+class Color {
+  final int _value;
+  int get value => _value;
+
+  const Color(this._value);
+  const Color.fromARGB(int a, int r, int g, int b) :
+    _value = (((a & 0xff) << 24) |
+              ((r & 0xff) << 16) |
+              ((g & 0xff) << 8) |
+              ((b & 0xff) << 0));
+
+}