Fix bugs found by Dart analyzer

sky/sdk/example/raw/painting.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' as sky;
 import 'dart:math' as math;
 import 'dart:typed_data';
 
 void beginFrame(double timeStamp) {
   sky.Size size = new sky.Size(sky.view.width, sky.view.height);
   sky.PictureRecorder recorder = new sky.PictureRecorder();
-  sky.Canvas canvas = new sky.Canvas(recorder, size);
+  sky.Canvas canvas = new sky.Canvas(recorder, sky.Point.origin & size);
 
   sky.Paint paint = new sky.Paint();
   sky.Point mid = size.center(sky.Point.origin);
   double radius = size.shortestSide / 2.0;
 
   canvas.drawPaint(new sky.Paint()..color = const sky.Color(0xFFFFFFFF));
 
   canvas.save();
 
   canvas.translate(-mid.x/2.0, sky.view.height*2.0);
@@ skipping 61 matching lines @@
   paint.setDrawLooper(builder.build());
   canvas.drawCircle(sky.Point.origin, radius, paint);
 
   sky.view.picture = recorder.endRecording();
 }
 
 void main() {
   sky.view.setFrameCallback(beginFrame);
   sky.view.scheduleFrame();
 }