Fix bugs found by Dart analyzer

sky/sdk/example/raw/painting.dart

Index: sky/sdk/example/raw/painting.dart
diff --git a/sky/sdk/example/raw/painting.dart b/sky/sdk/example/raw/painting.dart
index 90a7b467ad4977611f849d7ec2bf6174cf501f1e..2b3453201611c012ae140f48ce5c551f54179629 100644
--- a/sky/sdk/example/raw/painting.dart
+++ b/sky/sdk/example/raw/painting.dart
@@ -9,7 +9,7 @@ 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);