The Islanders allowed the go-ahead goal with 1:05 left against Calgary on Sunday night, so now they had to press six-on-five for the tying score. Goalie Jaroslav Halak was on the bench, but so was the creative kid who already had proved to be a playmaker with the puck on his stick. Coach Doug Weight had his reasons for turning rookie sensation Mathew Barzal into a Barclays Center spectator. But the tying score never came, and the Flames emerged with a 3-2 win. So how did the 20-year-old second-line center react to sitting? With maturity and accountability. “I’ll own up to it,” Barzal told Newsday after practice Monday at Northwell Health Ice Center. “I didn’t have a great game. I didn’t deserve to be out there late in the game. He made a good coaching move. I’ve got to hold myself accountable. I’m not going to sit here and pout and be upset that I wasn’t out there. I know I’ve got to play better.” Barzal also lost a little power-play time in the third period. He said the issue “was just more engaging in battles and whatnot. I was playing a little bit soft, not soft, but slow. A big part of my game is just playing down low and battles and winning that kind of stuff. So I wasn’t doing that [Sunday].” It’s just that the prime Calder Trophy contender has become such a threat. Barzal has 16 goals and leads all NHL rookies in assists with 43 and points with 59. He’s second in assists and third in points on the Islanders and had five assists in his previous game.