There is a small part of Jerry Dipoto that relishes Colt Emerson’s recent struggles in Triple-A. There is also a small part of Dipoto that wishes Kade Anderson’s run of unprecedented dominance in Double-A would detour into a dead end.  Prospects, you see, are supposed to be challenged. Some might suggest they even  need  to struggle before reaching the majors. Baseball isn’t supposed to be as easy as Anderson, the Mariners’ No. 1 pitching prospect, has made it appear in his first month as a professional.  It is supposed to be as much of a grind as it has been for Emerson, the Mariners’ No. 1 overall prospect. How young ballplayers respond to such challenges is often a strong indicator of their long-term outlook, Dipoto has learned. “The beauty of development is that there doesn’t have to be a rush,” Dipoto said. “There is no end game. There is no promotion date.…