Expand overloaded methods and optional parameters in the html library.

tests/html/canvasrenderingcontext2d_test.dart

Index: tests/html/canvasrenderingcontext2d_test.dart
diff --git a/tests/html/canvasrenderingcontext2d_test.dart b/tests/html/canvasrenderingcontext2d_test.dart
index 1aa0148cd6cc11791ede2e4b3aef3be88e181e1a..63d472b866b86233cb51587770ff08b1344b559c 100644
--- a/tests/html/canvasrenderingcontext2d_test.dart
+++ b/tests/html/canvasrenderingcontext2d_test.dart
@@ -668,5 +668,10 @@ main() {
       expect(other.width, 15);
       expect(other.height, 15);
     });
+
+    test('createPattern', () {
+      var pattern = context.createPattern(new CanvasElement(), '');
+      var pattern2 = context.createPatternFromImage(new ImageElement(), '');
+    });
   });
 }