If mount on tripod.. maybe very slight difference at 100% crop.
http://photozone.de/canon-eos/160-canon-ef-50mm-f18-ii-test-report--review?start=1
http://www.photozone.de/canon-eos/181-canon-ef-s-18-55mm-f35-56-is-test-report--review?start=1
Consider this.. at 50mm, take a picture with the EF50 @ F2, 1/40sec, compared to EF18-55IS @ F5.6, 1/5sec, I would say the 18-55mm will be sharper (assuming no camera shake). The 18-55 have 4th generation IS, so 1/5sec with IS would be similar to 1/40sec w/o IS.
If using large apertures, the EF50 F1.8 is not very sharp. At smaller apertures (F5.6 for e.g.), you need similar shutter speed as the 18-55mm, in which case, the 18-55IS has an advantage with IS.
The above comments is based on practical usage in reality. If the lens is always mount on tripod, all the above can be ignored.