“Sing Street” is an undercooked musical (play with music, really) adaption of the 2016 hit indie Irish film.  A sweet coming-of-age story with a gorgeous heart, the stage version gets none of the charm or quirkiness of the film right, and the thin, surface-deep story does not sustain its length.  Still, it remains a promising property that has the potential to be a knockout on stage, perhaps after more time in development.