Dick Barnett, who helped propel the Knicks to their glory days in the 1970s with his strange jump-shooting style, and who played on the only two N.B.A. championship teams in the Knicks’ history, died in his sleep last night in Largo, Fla. He was 88. The Knicks announced the death, at an assisted living facility, on social media on Sunday, soon after a dramatic victory in a first-round playoff series against the Detroit Pistons. Danielle Naassana, a producer of “The Dream Whisperer,” a PBS documentary about Barnett and his college career that came out last year, said he had become increasingly frail in recent years but did not appear to have a fatal illness.