Add an optional argument to HTMLCanvasElement.toDataURL.

lib/dom/dart2js/dom_dart2js.dart

Index: lib/dom/dart2js/dom_dart2js.dart
diff --git a/lib/dom/dart2js/dom_dart2js.dart b/lib/dom/dart2js/dom_dart2js.dart
index 0ff64956d5d7c01649e168865da1677ef898e362..89abd9307db085741efe076f6df195df7dd90d17 100644
--- a/lib/dom/dart2js/dom_dart2js.dart
+++ b/lib/dom/dart2js/dom_dart2js.dart
@@ -2821,7 +2821,7 @@ class _HTMLCanvasElementJs extends _HTMLElementJs implements HTMLCanvasElement n
 
   Object getContext(String contextId) native;
 
-  String toDataURL(String type) native;
+  String toDataURL(String type, [num quality]) native;
 }
 
 class _HTMLCollectionJs extends _DOMTypeJs implements HTMLCollection native "*HTMLCollection" {
@@ -15539,7 +15539,7 @@ interface HTMLCanvasElement extends HTMLElement {
 
   Object getContext(String contextId);
 
-  String toDataURL(String type);
+  String toDataURL(String type, [num 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