Make use of eslint-plugin-flowtype's case exhaustion in switch statements

[hawkrives]

https://github.com/mkscrg/eslint-plugin-flowtype/blob/768e7910b68eea8dbc6129862d6e6fbc0ebe03c6/.README/rules/no-unused-expressions.md

Essentially, anywhere a switch statement has an enum of the allowed types, we should typecast the expression as (empty), which is a special flow type which assures us that there are no more cases to be had.

[hawkrives]

Also use gajus/eslint-plugin-flowtype#236 (no-mutable-array)

[hawkrives]

Meh