import() to stage 3, private fields to stage 2; per 2016.12.01 TC39 · tc39/proposals@ab0c049
https://github.com/tc39/proposals/commit/ab0c049f7ef87a69d7fa3478e8e7a1ed035e9838
https://github.com/tc39/proposals/commit/ab0c049f7ef87a69d7fa3478e8e7a1ed035e9838