Fix bugs found by Dart analyzer

sky/sdk/example/raw/spinning_arabic.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" as math;
 import 'dart:sky';
 
 double timeBase = null;
 LayoutRoot layoutRoot = new LayoutRoot();
 
 void beginFrame(double timeStamp) {
   if (timeBase == null)
     timeBase = timeStamp;
   double delta = timeStamp - timeBase;
   PictureRecorder recorder = new PictureRecorder();
-  Canvas canvas = new Canvas(recorder, new Size(view.width, view.height));
+  Canvas canvas = new Canvas(recorder, new Rect.fromLTWH(0.0, 0.0, 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));
 
   double sin = math.sin(delta / 200);
   layoutRoot.maxWidth = 150.0 + (50 * sin);
   layoutRoot.layout();
 
   canvas.translate(layoutRoot.maxWidth / -2.0, (layoutRoot.maxWidth / 2.0) - 125);
@@ skipping 12 matching lines @@
   block.style['direction'] = 'rtl';
   block.style['unicode-bidi'] = 'plaintext';
   block.appendChild(arabic);
   block.appendChild(more);
 
   layoutRoot.rootElement = block;
 
   view.setFrameCallback(beginFrame);
   view.scheduleFrame();
 }