Update context2d.arc to take optional anticlockwise parameter

tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate

Index: tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate b/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
index 1e9ba85ac4d65731890463c795b0fde1e9129f9b..2c6ca71a56e177dc10a7a5223b32a3b761aca24e 100644
--- a/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
+++ b/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
@@ -43,6 +43,13 @@ $!MEMBERS
     this.strokeStyle = 'hsla($h, $s%, $l%, $a)';
   }
 
+  @DomName('CanvasRenderingContext2D.arc')
+  void arc(num x,  num y,  num radius,  num startAngle, num endAngle,

[sra1, 2013/04/09 22:30:58]

This won't inline very well for dart2js.
Is it possible to make arc the native method?
The 6th argument is optional in webkit and w3 IDL.

Is there some browser that either does the wrong thing?
If the problem is Dartium, we can use $if DART2JS

[Andrei Mouravski, 2013/04/09 23:33:24]

The argument is optional in the spec, too:
http://www.w3.org/html/wg/drafts/2dcontext/html5_canvas/#dom-context-2d-arc

+      [bool anticlockwise: false]) {
+    $dom_arc(x, y, radius, startAngle, endAngle, anticlockwise);
+  }
+
+
 $if DART2JS
   @DomName('CanvasRenderingContext2D.lineDashOffset')
   num get lineDashOffset => JS('num',