Some HTML API cleanup

tests/html/canvas_test.dart

 #library('CanvasTest');
 #import('../../lib/unittest/unittest.dart');
 #import('../../lib/unittest/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;
   document.body.nodes.add(canvas);
 
-  context = canvas.getContext('2d');
+  context = canvas.context2d;
 
   useHtmlConfiguration();
   test('FillStyle', () {
     context.fillStyle = "red";
     context.fillRect(10, 10, 20, 20);
 
     Uint8ClampedArray data = context.getImageData(0, 0, width, height).data;
     checkPixel(data, 0, [0, 0, 0, 0]);
     checkPixel(data, 9 + width * 10, [0, 0, 0, 0]);
     checkPixel(data, 10 + width * 10, [255, 0, 0, 255]);
@@ skipping 53 matching lines @@
   });
 }
 
 void checkPixel(Uint8ClampedArray data, int offset, List<int> rgba)
 {
   offset *= 4;
   for (var i = 0; i < 4; ++i) {
     Expect.equals(rgba[i], data[offset + i]);
   }
 }