Move sky/examples to sky/sdk/lib/example, and code changes to support that change. Fixes T277.

sky/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';
-import 'dart:math' as math;
-
-double timeBase = null;
-
-void beginFrame(double timeStamp) {
-  tracing.begin('beginFrame');
-  if (timeBase == null)
-    timeBase = timeStamp;
-  double delta = timeStamp - timeBase;
-  PictureRecorder recorder = new PictureRecorder();
-  Canvas canvas = new Canvas(recorder, view.width, view.height);
-  canvas.translate(view.width / 2.0, view.height / 2.0);
-  canvas.rotate(math.PI * delta / 1800);
-  canvas.drawRect(new Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
-                  new Paint()..color = const Color.fromARGB(255, 0, 255, 0));
-  view.picture = recorder.endRecording();
-  view.scheduleFrame();
-  tracing.end('beginFrame');
-}
-
-void main() {
-  view.setBeginFrameCallback(beginFrame);
-  view.scheduleFrame();
-}