Temporary fix for CanvasRenderingContext2D.arc

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;
 
 abstract class CanvasRenderingContext {
   CanvasElement get canvas;
 }
 
@@ skipping 32 matching lines @@
    * [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,
       [bool anticlockwise = false]) {
+    // TODO(terry): This should not be needed: dartbug.com/20939.
+$if DART2JS
+    JS('void', '#.arc(#, #, #, #, #, #)', this, x, y, radius, startAngle,
+       endAngle, anticlockwise);
+$else
     _arc(x, y, radius, startAngle, endAngle, anticlockwise);
+$endif
   }
 
   /**
    * Draws an image from a CanvasImageSource to an area of this canvas.
    *
    * The image will be drawn to an area of this canvas defined by
    * [destRect]. [sourceRect] defines the region of the source image that is
    * drawn.
    * If [sourceRect] is not provided, then
    * the entire rectangular image from [source] will be drawn to this context.
@@ skipping 290 matching lines @@
     JS('void', '#.fill(#)', this, winding);
   }
 $endif
 
   /** Deprecated always returns 1.0 */
   @DomName('CanvasRenderingContext2D.webkitBackingStorePixelRation')
   @Experimental()
   @deprecated
   double get backingStorePixelRatio => 1.0;
 }