A helper to check the type compatibility.
import type {IsCompat} from 'flown'
;(true: IsCompat<2, 2>)
;(false: IsCompat<2, 3>)
;(true: IsCompat<{a: number}, {a: number}>)
;(true: IsCompat<{a: number}, {|a: number|}>)
;(true: IsCompat<{a: number}, {a: number, b?: number}>)