Expand overloaded methods and optional parameters in the html library.

tests/html/canvasrenderingcontext2d_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 canvas_rendering_context_2d_test; import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 import 'dart:math';
 
 // Some rounding errors in the browsers.
@@ skipping 650 matching lines @@
 
     test('createImageData', () {
       var imageData = context.createImageData(15, 15);
       expect(imageData.width, 15);
       expect(imageData.height, 15);
 
       var other = context.createImageDataFromImageData(imageData);
       expect(other.width, 15);
       expect(other.height, 15);
     });
+
+    test('createPattern', () {
+      var pattern = context.createPattern(new CanvasElement(), '');
+      var pattern2 = context.createPatternFromImage(new ImageElement(), '');
+    });
   });
 }