The ornamental design for a flagstopper, as shown and described.
FIG. 1 is a perspective view of a flag stopper showing my new design, as attached to an exemplary flagpole;
FIG. 2 is a front elevation view thereof with the exemplary flagpole removed, the rear elevation view being a mirror image of the front elevation view;
FIG. 3 is a right side elevation view thereof, the left side elevation view being a mirror image of the right side elevation view;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a perspective view of an alternative embodiment of the flag stopper; and,
FIG. 7. is a front elevation view of the alternative embodiment of FIG. 6.
The broken lines in the figures represent environmental subject matter which forms no part of the claimed design.