The ornamental design for a flask with cap, as shown and described.
FIG. 1 is a perspective view showing the new design for the flask with cap;
FIG. 2 is a front view of the flask with cap shown in FIG. 1;
FIG. 3 is a rear view of the flask with cap shown in FIG. 1;
FIG. 4 is a right side view of the flask with cap shown in FIG. 1;
FIG. 5 is a top view of the flask with cap shown in FIG. 1;
FIG. 6 is a left side view of the flask with cap shown in FIG. 1; and,
FIG. 7 is a bottom view of the flask with cap shown in FIG. 1.
The broken line shown in FIG. 7 is included for the purpose of illustrating the portion of the bottom surface that forms no part of the claimed design.