message: Fix `Array#flat` typing to work on `ReadonlyArray`
diff: interface Array<T> {
  - flat<U>(this: U[][], depth?: 1): U[];
  + flat<U>(this: ReadonlyArray<U>[], depth?: 1): U[];
  flatMap<U>(
  callbackfn: (value: T, index: number, array: T[]) => ReadonlyArray<U> | U,
  thisArg?: this
  