Rename context2d to context2D.

tests/html/url_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library url_test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   Blob createImageBlob() {
     var canvas = new CanvasElement();
     canvas.width = 100;
     canvas.height = 100;
 
-    var context = canvas.context2d;
+    var context = canvas.context2D;
     context.fillStyle = 'red';
     context.fillRect(0, 0, canvas.width, canvas.height);
 
     var dataUri = canvas.toDataUrl('image/png');
     var byteString = window.atob(dataUri.split(',')[1]);
     var mimeString = dataUri.split(',')[0].split(':')[1].split(';')[0];
 
     var arrayBuffer = new ArrayBuffer(byteString.length);
     var dataArray = new Uint8Array.fromBuffer(arrayBuffer);
     for (var i = 0; i < byteString.length; i++) {
@@ skipping 36 matching lines @@
       img.onLoad.listen((_) {
         guardAsync(() {
           expect(true, isFalse, reason: 'URL should not have loaded.');
         });
       });
       img.src = url;
     });
 
   });
 }