added missing export statement to PolarGridHelper.js

src/extras/helpers/PolarGridHelper.js

@@ -90,4 +90,6 @@ function PolarGridHelper( radius, radials, circles, divisions, color1, color2 )
 }
 
 PolarGridHelper.prototype = Object.create( LineSegments.prototype );
-PolarGridHelper.prototype.constructor = PolarGridHelper;
+PolarGridHelper.prototype.constructor = PolarGridHelper;
+
+export { PolarGridHeler };