The ornamental design for a flashlight end cap, as shown and described.
FIG. 1 is a perspective view of a flashlight end cap embodying my new design, with the flashlight shown exploded and in broken lines for illustrative purposes only, the figure showing the end cap positioned for insertion into an end of the flashlight.
FIG. 2 is a front view of the end cap of FIG. 1.
FIG. 3 is a bottom view of the end cap of FIG. 1; and,