Is there a bona fide case proving dogs should not eat chocolate?

I have heard this all my life that chocolate is not good for dogs. I had a toy poodle that ate Hershey’s Chocolate Kisses like they were going out of style. He had his own bag in his stocking, each year for Christmas. He died, but not from chocolate. Question: is there a bona fide actual proven case that chocolate has taken the life of a dog, in the past? If so, I would love to read the research results. Thanks.