Update context2d.arc to take optional anticlockwise parameter

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

 // 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
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
 
   /**
@@ skipping 25 matching lines @@
   /**
    * Sets the color used for stroking shapes.
    * [h] is in degrees, 0-360.
    * [s], [l] are in percent, 0-100.
    * [a] is 0-1.
    */
   void setStrokeColorHsl(int h, num s, num l, [num a = 1]) {
     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',
       '#.lineDashOffset || #.webkitLineDashOffset', this, this);
 
   @DomName('CanvasRenderingContext2D.lineDashOffset')
   void set lineDashOffset(num value) => JS('void', 
       'typeof #.lineDashOffset != "undefined" ? #.lineDashOffset = # : '
       '#.webkitLineDashOffset = #', this, this, value, this, value);
 $endif
 }