I've had experience of chocolate poisoning with Jess. She polished off half a bar of Green and Black's 70% when she was about 2, and she was pretty poorly at the time. She got better but the vet gave me a terrible row for letting it happen, so we both came home with our tails between our legs!
HOWEVER - Jess became extremely sensitised to chocolate, so that when she licked out a bowl (at the dishwasher, cocker-fashion) which had contained some chocolate flavoured ice-cream, she became very poorly indeed and had to be put on a drip and we thought we'd lose her. Since then, I've been paranoid about dogs and chocolate.