See my caution below, and proceed at your own risk!
I think that methylene chloride might do the trick, though I've never tried it in a situation such as you describe. I found it (in the NW U.S.) as "Ambroid ProWeld" in a hobby shop. (I understand that what I have is not 100% methylene chloride, and that a stronger solution, if found, would be better.) I've used it for several things HP-related.
You need to hold the pieces to be glued together closely, and let the glue be drawn into the joint by capillary action. The pieces must be held in close approximation for some time. (At least an hour?) The methylene chloride (a highly volatile aromatic hydrocarbon, if I've got my chemistry straight) melts the plastic and fuses the pieces together. A few of the things on which I have used it (including an HP-97 battery compartment door, broken right in half) seem as strong as new. (But it hasn't worked on everything.)
I worry, however about applying it to anything attached to a calculator, and to anything that needs to move again someday. If at all possible, take the switch out of the calculator before using any glue.