Nets collapse down stretch in loss to Mavericks to spoil another big Michael Porter Jr. effort

DALLAS — Young teams have to learn how to win in the endgame. Friday’s fourth quarter folding was an example of that.  The Nets lost 119-111 to the Mavericks before 19,032 at American Airlines Center, a game where they couldn’t buy a basket or a rebound down the stretch.  Brooklyn shot just 6-for-21 overall and…