use .append instead of .nodes.add

tests/html/canvas_test.dart

 library CanvasTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   CanvasElement canvas;
   CanvasRenderingContext2D context;
   int width = 100;
   int height = 100;
 
   canvas = new CanvasElement(width:width, height:height);
-  document.body.nodes.add(canvas);
+  document.body.append(canvas);
 
   context = canvas.context2D;
 
   useHtmlConfiguration();
   test('CreateImageData', () {
     ImageData image = context.createImageData(canvas.width,
                                               canvas.height);
     List<int> data = image.data;
 
     expect(data, hasLength(40000));
@@ skipping 25 matching lines @@
   });
 }
 
 void checkPixel(List<int> data, int offset, List<int> rgba)
 {
   offset *= 4;
   for (var i = 0; i < 4; ++i) {
     expect(data[offset + i], equals(rgba[i]));
   }
 }