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

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 62fa29ded7d64d48d552b7171dc507d499826306..db9c4d25536706f53cf5eb6a7fb8be65bbb8abf4 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -1091,7 +1091,7 @@ class CanvasElement extends _HTMLElement implements CanvasImageSource {
    */
   @DomName('HTMLCanvasElement.toDataURL')
   @DocsEditable()
-  String toDataUrl(String type, [num quality]) native "HTMLCanvasElement_toDataURL_Callback";
+  String $dom_toDataUrl(String type, [num quality]) native "HTMLCanvasElement_toDataURL_Callback";
 
   @DomName('HTMLCanvasElement.onwebglcontextlost')
   @DocsEditable()
@@ -1127,6 +1127,9 @@ class CanvasElement extends _HTMLElement implements CanvasImageSource {
     }
     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