Fix bugs found by Dart analyzer

sky/sdk/example/raw/spinning_image.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';
 
 import 'package:sky/mojo/net/image_cache.dart' as image_cache;
 
 double timeBase = null;
 
 Image image = null;
 String url1 = "https://www.dartlang.org/logos/dart-logo.png";
 String url2 = "http://i2.kym-cdn.com/photos/images/facebook/000/581/296/c09.jpg";
 
 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, Point.origin & new Size(view.width, view.height));
   canvas.translate(view.width / 2.0, view.height / 2.0);
   canvas.rotate(math.PI * delta / 1800);
   canvas.scale(0.2, 0.2);
   Paint paint = new Paint()..color = const Color.fromARGB(255, 0, 255, 0);
 
   // Draw image
   if (image != null)
     canvas.drawImage(image, new Point(-image.width / 2.0, -image.height / 2.0), paint);
 
   // Draw cut out of image
@@ skipping 32 matching lines @@
   }
 
   return false;
 }
 
 void main() {
   image_cache.load(url1).then(handleImageLoad);
   view.setEventCallback(handleEvent);
   view.setFrameCallback(beginFrame);
 }