Fix bugs found by Dart analyzer

sky/sdk/example/raw/painting_node.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";
 import 'dart:sky';
 
 PaintingNode paintingNode = null;
 Picture draw(int a, int r, int g, int b) {
   Rect bounds = new Rect.fromLTRB(0.0, 0.0, view.width, view.height);
   Size size = new Size(view.width, view.height);
 
   PictureRecorder recorder = new PictureRecorder();
   Canvas canvas = new Canvas(recorder, bounds);
   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);
 
   if (paintingNode == null) {
     paintingNode = new PaintingNode();
     Paint innerPaint = new Paint()..color = new Color.fromARGB(a, 255 - r, 255 - g, 255 - b);
     PictureRecorder innerRecorder = new PictureRecorder();
     Canvas innerCanvas = new Canvas(innerRecorder, bounds);
     innerCanvas.drawCircle(size.center(Point.origin), radius * 0.5, innerPaint);
 
     paintingNode.setBackingDrawable(innerRecorder.endRecordingAsDrawable());
   }
-  canvas.drawPaintingNode(paintingNode);
+  canvas.drawPaintingNode(paintingNode, Point.origin);
 
   return recorder.endRecording();
 }
 
 bool handleEvent(Event event) {
   if (event.type == "pointerdown") {
     view.picture = draw(255, 0, 0, 255);
     view.scheduleFrame();
     return true;
   }
 
   if (event.type == "pointerup") {
     view.picture = draw(255, 0, 255, 0);
     view.scheduleFrame();
     return true;
   }
 
   return false;
 }
 
 void main() {
   view.picture = draw(255, 0, 255, 0);
   view.scheduleFrame();
 
   view.setEventCallback(handleEvent);
 }