Result type in javascript with flow

Result.js

/**
* @flow
* @providesModule Result
*/
'use strict'

export const SuccessType = 'SuccessType'
export const ErrorType = 'ErrorType'

type SuccessT<T> = T & {
  kind: 'SuccessType'
}

type ErrorT<T> = {
  kind: 'ErrorType',
  error: T
}

export type Result<S, E> = SuccessT<S> | ErrorT<E>

export function Success<T>(object: T): SuccessT<T> {
  return {
    kind: SuccessType,
    ...object
  }
}

export function Error<T>(error: T): ErrorT<T> {
  return {
    kind: ErrorType,
    error: error
  }
}

Don't you lose the prototype of the success value with this?