Updating Canvas test to use the most common syntax.

tests/html/canvas_test.dart

Index: tests/html/canvas_test.dart
diff --git a/tests/html/canvas_test.dart b/tests/html/canvas_test.dart
index 0f9deaeb0578f200e2398516e2b7322fa13ae7b0..3f6e700a1a9243b41217ce6621ef75ea22d5c662 100644
--- a/tests/html/canvas_test.dart
+++ b/tests/html/canvas_test.dart
@@ -9,9 +9,7 @@ main() {
   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;