Disney-Marvel’s “The Avengers” is well on its way to scoring the best domestic sophomore session of all time, with early B.O. estimates forecasting an impressive $85 million-plus for the three-day weekend.The second-weekend benchmark was previously held by Fox’s “Avatar” at $75.5 million. “I wouldn’t be surprised if that number ($85 million) goes up,” one exec warned, noting word-of-mouth combined with Mother’s Day could see the superhero actioner climb to $95 million — though it’s too early to tell. “Avengers” is rapidly approaching the $1 billion mark worldwide, having already eclipsed the global cumes of Marvel properties “Iron Man,” “Iron Man 2,” “Captain America,” and “Thor.” Meanwhile, despite lukewarm reviews, Warner Bros.’ “Dark Shadows” is doing moderate biz in second place. The Tim Burton-Johnny Depp pic should open from $30 million to $35 million, per Friday estimates, which includes $550,000 from midnight screenings Thursday. Based on the cult soap opera from the late 60s, “Dark Shadows” is the eighth collaboration between Depp and Burton and co-stars Michelle Pfeiffer, Eva Green and Chloe Moretz.