Flow is failing to infer instanceof checks

doesntWork.js

/* @flow */

function Children (children : Iterable<any>) {
  this.children = Array.from(children)
  return this
}

Children.prototype = {
  toArray () : any[] {
    return this.children
  },
  get length () : number {
    return this.children.length
  },
  [Symbol.iterator] () : Iterator<any> {
    return this.children[Symbol.iterator]()
  }
}

const asChildren = (children : Iterable<any> = []) : Children =>
  isChildren(Children)
    ? children
    : new Children(children)

const isChildren =
  (children : ?any) : boolean =>
    children instanceof Children

works.js

/* @flow */

function Children (children : Iterable<any>) {
  this.children = Array.from(children)
  return this
}

Children.prototype = {
  toArray () : any[] {
    return this.children
  },
  get length () : number {
    return this.children.length
  },
  [Symbol.iterator] () : Iterator<any> {
    return this.children[Symbol.iterator]()
  }
}

const asChildren = (children : Iterable<any> = []) : Children =>
  children instanceof Children
    ? children
    : new Children(children)

const isChildren =
  (children : ?any) : boolean =>
    children instanceof Children