Add a simple_render_tree example

This example shows how to draw a circle using subclasses of RenderNode.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1144193004

examples/raw/simple_render_tree.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:math';
+import 'dart:sky';
+import 'package:sky/framework/layout2.dart';
+
+class RenderBlueCircle extends RenderBox {
+  void paint(RenderNodeDisplayList canvas) {
+    double radius = min(width, height) * 0.45;
+    Paint paint = new Paint()..setARGB(255, 0, 255, 255);
+    canvas.drawCircle(width / 2, height / 2, radius, paint);
+  }
+}
+
+void main() {
+  RenderView renderView = new RenderView(root: new RenderBlueCircle());
+  renderView.layout(newWidth: view.width, newHeight: view.height);
+  renderView.paintFrame();
+}