You’d think the death by suicide of a lonely teen would be a total downer. In Dear Evan Hansen, the play that opened at Broadway’s Music Box Theater on Sunday night after an ecstatic off Broadway run, we meet the troubled boy, Connor (Mike Faist), early on. He’s a bully and picks on the play’s protagonist before the tragic deed, and the rest of this exuberant, fresh, musical is about how, in the age of the Internet, something big is made of something so unknowable and private as this young man’s demise. Ben Platt plays Evan Hansen. An actor of mega talent who made a huge impression in the Pitch Perfect movies, Platt has many fans who clapped hard the minute he took stage, in his bed writing a letter of affirmation to himself: “Dear Evan Hansen,” it starts.