Port spinning_square to SkyView universe

This demo shows how to create a spinning square using the lowest-level APIs. We
should probably create more spinning square demos at the other layers of the
system.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1147393002

examples/raw/spinning_square.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';
+
+double timeBase = null;
+
+void beginFrame(double timeStamp) {
+  if (timeBase == null)
+    timeBase = timeStamp;
+  double delta = timeStamp - timeBase;
+  PictureRecorder canvas = new PictureRecorder(view.width, view.height);
+  canvas.translate(view.width / 2.0, view.height / 2.0);
+  canvas.rotateDegrees(delta / 10);
+  canvas.drawRect(new Rect()..setLTRB(-100.0, -100.0, 100.0, 100.0),
+                  new Paint()..setARGB(255, 0, 255, 0));
+  view.picture = canvas.endRecording();
+  view.scheduleFrame();
+}
+
+void main() {
+  view.setBeginFrameCallback(beginFrame);
+  view.scheduleFrame();
+}