Fix bugs found by Dart analyzer

sky/sdk/example/raw/shadow.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';
 
 void beginFrame(double timeStamp) {
   double size = 100.0;
   PictureRecorder recorder = new PictureRecorder();
-  Canvas canvas = new Canvas(recorder, new Size(view.width, view.height));
+  Canvas canvas = new Canvas(recorder, new Rect.fromLTWH(0.0, 0.0, view.width, view.height));
   canvas.translate(size + 10.0, size + 10.0);
 
   Paint paint = new Paint();
   paint.color = const Color.fromARGB(255, 0, 255, 0);
   var builder = new LayerDrawLooperBuilder()
     // Shadow layer.
     ..addLayerOnTop(
         new DrawLooperLayerInfo()
           ..setPaintBits(PaintBits.all)
           ..setOffset(const Offset(5.0, 5.0))
@@ skipping 10 matching lines @@
   canvas.drawPaint(
       new Paint()..color = const Color.fromARGB(255, 255, 255, 255));
   canvas.drawRect(new Rect.fromLTRB(-size, -size, size, size), paint);
   view.picture = recorder.endRecording();
 }
 
 void main() {
   view.setFrameCallback(beginFrame);
   view.scheduleFrame();
 }