The ornamental design for a handlebarstem for a bicycle, as shown and described.
FIG. 1 is a front perspective view of a handlebar stem for a bicycle showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a front view thereof; and,
FIG. 8 is a bottom view thereof.
The broken line showing of the various unshaded regions of the handlebar stem for a bicycle illustrates environmental structure and forms no part of the claimed design.