message: duck type UnderlineNavLink; nix circular dep in
diff: import React from 'react'
  import PropTypes from 'prop-types'
  import classnames from 'classnames'
  -import UnderlineNavLink from './UnderlineNavLink'
  import {mapWhitespaceProps} from './props'
  export const ITEM_CLASS = 'UnderlineNav-item no-underline'
  @@ -13,7 +12,7 @@ export default function UnderlineNav(props) {
  const classes = classnames(className, 'UnderlineNav', align && `UnderlineNav--${align}`, full && 'UnderlineNav--full')
  const mappedChildren = React.Children.map(children, child => {
  - if (child.type === UnderlineNavLink) {
  + if (child.type.displayName === 'UnderlineNavLink') {
  return child
  }
  