In Musks's 17 Sep 2018 presentation from Hawthorne, announcing the #dearMoon lunar flyby, he also identified several precursor steps:
2019: hopper flights from Texas facility
2020: BFS high altitude flight tests
2020: BFR booster tests
2021 (aspirational): multiple unmanned orbital flights prior to first crewed flight
~2022-2023: probable unmanned lunar flyby prior to #dearMoon mission