Currently we have a 22mm f/2.0 and a 28mm f/3.5 for EF-M.
The stunning lack of new EF-M prime lenses on a system that is actually selling very well (especially in Asia) tells me one of two things:
a) They're assuming that people are happy with the lighter EF primes and the EF/EF-M adaptor (which is true in many cases - I see no need for an EF-M 50mm 1.8 for example when the combination with the mount adaptor and the standard EF 50mm is so light.
Certainly the combinations of EF 40mm f/2.8, 50mm f/1.8 and to a lesser extent the 50mm f/1.4, 85mm f/1.8 and the 24/28/35 IS trinity with the adaptor give a great variety of relatively compact lenses. They are missing a lightweight wide-angle prime however.
b) They're waiting to figure out what they want to do with full-frame. If, for example, they were to do an EF-M 50mm f/1.8 then they might as well do it with a full-frame image circle. But how do Canon market a full frame EF-M lenses when you haven't got a system yet that can use them at full frame? Answer: you hold on and wait until you've decided what you want to do.