Make type parameter in toDataUrl have an optional default value as per the spec.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 20a57082b43f1cbace3fb1c426058a4d68d834f9..f5ed20735ff73a8f7a45a9c1000d290119521ab0 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -906,7 +906,7 @@ class CanvasElement extends _HTMLElement implements CanvasImageSource native "HT
    */
   @DomName('HTMLCanvasElement.toDataURL')
   @DocsEditable()
-  String toDataUrl(String type, [num quality]) native;
+  String $dom_toDataUrl(String type, [num quality]) native;
 
   @DomName('HTMLCanvasElement.onwebglcontextlost')
   @DocsEditable()
@@ -943,6 +943,9 @@ class CanvasElement extends _HTMLElement implements CanvasImageSource native "HT
     }
     return context;
   }
+
+  String toDataUrl([String type = 'image/png', num quality]) => 
+      $dom_toDataUrl(type, quality);
 }
 // 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