Attempt to write a RenderShadowedBox

The goal of this is to have a Renderer-layer abstraction
around the Sky Engine c++ apis for drawing a shadow.

R=mpcomplete@chromium.org

Review URL: https://codereview.chromium.org/1146893004

examples/raw/shadowed_box.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.
+
+import 'dart:sky';
+import 'package:sky/framework/app.dart';
+import 'package:sky/framework/rendering/box.dart';
+import 'package:sky/framework/rendering/flex.dart';
+
+AppView app;
+
+void main() {
+  var coloredBox = new RenderDecoratedBox(
+    decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFF00))
+  );
+  var shadow = const BoxShadow(
+      color: const Color(0xFFEEEEEE), offset: const Size(5.0, 5.0), blur: 5.0);
+  var shadowBox = new RenderShadowedBox(shadow: shadow, child: coloredBox);
+  var paddedBox = new RenderPadding(padding: const EdgeDims.all(30.0),
+      child: shadowBox);
+  app = new AppView(paddedBox);
+}