Change // @sourceMappingURL to // #sourceMappingURL

See https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.lmz475t4mvbx
Make wrapup and output emitters
Better logging and error handling