fix error in arcTo.

Runtime/ui/painting/path.cs

             }
 
             var points = TessellationGenerator.tessellateBezier(x1, y1, x2, y2, x3, y3, x4, y4, this._tessTol);
-            
+            D.assert(points.Count > 0);            
             points[points.Count - 1].flags = flags;
             foreach (var point in points) {
                 this._addPoint(point);