This I'll be curious to see. Most of the Stihl plastic components are straight nylon or glass filled nylon. Nylon is tough to weld as it is high melting, and with glass in it, you will never get a full strength joint where you join it as the glass fiber won't bridge the joint, assuming you successfully can weld it at all. It is also very solvent resistant, and as a result hard to glue (successfully).
If the break is in a low-stress area, you might get away with it. But usually breaks are in high stress areas (that's why it broke there...) unless it was the old tree-drop or truck crunch that got it.
-Dave