Add an optional argument to HTMLCanvasElement.toDataURL.

lib/html/dartium/html_dartium.dart

Index: lib/html/dartium/html_dartium.dart
diff --git a/lib/html/dartium/html_dartium.dart b/lib/html/dartium/html_dartium.dart
index 5e7c417b1da851a80be3fc7e1b7a2455bcacd242..5f6d135d0b2a4e0f14e6998b4afa5302b7b86712 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -8697,7 +8697,7 @@ class _HTMLCanvasElementImpl extends _HTMLElementImpl implements CanvasElement {
 
   Object getContext(String contextId) native "HTMLCanvasElement_getContext_Callback";
 
-  String toDataURL(String type) native "HTMLCanvasElement_toDataURL_Callback";
+  String toDataURL(String type, [num quality]) native "HTMLCanvasElement_toDataURL_Callback";
 
 
   CanvasRenderingContext2D get context2d() => getContext('2d');
@@ -24736,7 +24736,7 @@ interface CanvasElement extends Element default _Elements {
   Object getContext(String contextId);
 
   /** @domName HTMLCanvasElement.toDataURL */
-  String toDataURL(String type);
+  String toDataURL(String type, [num quality]);
 
   final CanvasRenderingContext2D context2d;
 }