Fix bugs found by Dart analyzer

sky/sdk/example/raw/hello_world.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';
 
 Picture draw(int a, int r, int g, int b) {
   Size size = new Size(view.width, view.height);
 
   PictureRecorder recorder = new PictureRecorder();
-  Canvas canvas = new Canvas(recorder, size);
+  Canvas canvas = new Canvas(recorder, Point.origin & size);
   double radius = size.shortestSide * 0.45;
 
   Paint paint = new Paint()..color = new Color.fromARGB(a, r, g, b);
   canvas.drawCircle(size.center(Point.origin), radius, paint);
   return recorder.endRecording();
 }
 
 bool handleEvent(Event event) {
   if (event.type == "pointerdown") {
     view.picture = draw(255, 0, 0, 255);
@@ skipping 15 matching lines @@
   return false;
 }
 
 void main() {
   print("Hello, world");
   view.picture = draw(255, 0, 255, 0);
   view.scheduleFrame();
 
   view.setEventCallback(handleEvent);
 }