Add “stopper” to Jacob deGrom’s burgeoning Mets résumé. In a solid rotation, minus Matt Harvey, it’s the rookie deGrom moving to the forefront. Or you could just call him de facto ace. On Tuesday, he filled that role nicely, with seven dominant innings that led to a 3-1 victory over the Mariners at Safeco Field, snapping the Mets’ three-game losing streak. The Mets are scuffling offensively to begin the second half, but deGrom made the lineup’s lapses easier to stomach for a night. Losing streak or no losing streak, deGrom (4-5) was determined to succeed. “I just went out with the same game plan, try to get strike one,” deGrom said after allowing one run on five hits over seven innings with one walk and seven strikeouts. “Get guys out early to try to stay in the game for awhile and I had some great plays behind me. They hit some balls hard right at people, but things just seemed to go my way tonight.” It was a third consecutive start for the right-hander in which he lasted at least seven innings and allowed one run or less. Over that stretch he is 3-0 with a 0.86 ERA with 26 strikeouts and three walks. “This guy is going to figure it out, and he’s going to be tough, really somebody to deal with in the future,” manager Terry Collins said. “He’s gotten his opportunity and he’s made the most of it.” Jeurys Familia worked a scoreless eighth — receiving help from Juan Lagares, who went racing toward the center-field fence to snare Endy Chavez’s drive leading off the inning — before Jenrry Mejia got his 12th save in 14 chances.