Dog ate chocolate

Make sure you get it checked by the vet. Although there may not be any reaction from the dog it can lead to long-term problems in the future. My Aunts dog use to eat anything and everything so be careful and vigilant. Teach your dog a 'leave it' cue to allow your dog to learn what as this will keep them safe. 

Suziehill,

                Absolutely get your dog checked out by your vet if you haven't already. Chocolate is incredibly toxic to dogs due to the chemical theobromine which can accumulate due to the inability to metabolize it. You mentioned your dog is a Yorkie which is a great concern to me due to their size and the amount of chocolate ingested. 
Your vet can administer a drug and activated charcoal to cause your dog to empty its stomach through vommitting and then the charcoal will deactivate toxins.

A tip for next time is if you have any sodium carbonate/ soda crystals (NOT BICARBONATE OF SODA) is to put a few crystals at the back of their mouth to induce vommitting (ONLY if you have spoken to your vet and they reccomend this before travelling for emergency treatment.)

Kind regards,

Joe