Chocolate is toxic to start with for dogs and for your dog the sudden influx of the sugar in the bar may be another big issue.
A lot depends on the size of the dog and the type of the chocolate how dangerous it is.
You can read here about the different toxic levels of different chocolate types and the amount that is toxic per pound of dog etc.http://www.talktothevet.com/ARTICLES/DOGS/chocolatetoxic.HTM
And another site here
http://www.dogownersdigest.com/news/library/chocolate-dog-poisoning.shtml
If your dog just ate it within the last 60 minutes you can get the dog to vomit it up which you can read about scrolling down on this page http://www.petplace.com/dogs/how-to-induce-vomiting-emesis-in-dogs/page1.aspx Do this only on an easily cleaned surface and some people have the dog stand in the tub to vomit so clean up is easy.
But with your dog's diabetes you may want in person vet care to help him with fluids and other supportive care and monitoring of sugar levels.
This site may help you locate an emergency vet if you need one
http://www.vetlocator.com/hotline.php
Hope this helps you!
Pet Health Care, Rescue,Train,Breed
30+yrs pet vet care & nursing, rescue, behavior & training, responsible breeding, small animal care