There was some sloppy play Sunday night by the Islanders that left coach Doug Weight frustrated after the game, but there was at least one thing that was clean, crisp and quick: Mathew Barzal. The Islanders’ 20-year-old center continued to break out, setting a single-game franchise rookie record with five assists to spark a 6-4 win over the Avalanche at Barclays Center. “His speed is unbelievable,” Dennis Seidenberg said. “He’s gliding. It’s not looking like he’s moving, but he’s just actually flying.” Two of Barzal’s assists were highlight-reel helpers on Jordan Eberle goals, fueling the Islanders (8-5-1) to their sixth win in the last eight games. “It was kind of a night where everything went right,” said Barzal, who became the first NHL rookie since 2011 to tally five assists. “I’m more confident every day. Nights like tonight don’t really hurt. Just trying to fit in and make an impact.” Nick Leddy added a pair of goals to go with an assist while fellow defensemen Scott Mayfield and Johnny Boychuk also scored. Mayfield completed the Gordie Howe hat trick with a goal, an assist and a fight. The Islanders continued to excel on the power play, as well. After starting the year 2-for-33 on the man-advantage, they have converted on nine of their last 16 chances. That included two more goals Sunday — both from Leddy — when they needed just 38 seconds on the power play to capitalize. While the Islanders built up a 6-2 lead by early in the third period, the Avalanche made the final margin look more respectable as Alexander Kerfoot scored two goals in the final 30 seconds.