Updating Canvas test to use the most common syntax.

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 Element.tag('canvas');
-  canvas.attributes['width'] = width;
-  canvas.attributes['height'] = height;
+  canvas = new CanvasElement(width:width, height:height);
   document.body.nodes.add(canvas);
 
   context = canvas.context2d;
 
   useHtmlConfiguration();
   test('CreateImageData', () {
     ImageData image = context.createImageData(canvas.width,
                                               canvas.height);
     Uint8ClampedArray data = image.data;
 
     expect(data, hasLength(40000));
     checkPixel(data, 0, [0, 0, 0, 0]);
     checkPixel(data, width * height - 1, [0, 0, 0, 0]);
 
     data[100] = 200;
     expect(data[100], equals(200));
   });
 }
 
 void checkPixel(Uint8ClampedArray data, int offset, List<int> rgba)
 {
   offset *= 4;
   for (var i = 0; i < 4; ++i) {
     expect(data[offset + i], equals(rgba[i]));
   }
 }