message: properly derive borderColor in Caret
diff: @@ -2,6 +2,12 @@ import React from 'react'
  import PropTypes from 'prop-types'
  import theme, {colors} from './theme'
  +const borderColors = Object.keys(colors.border)
  +
  +function getBorderColor(key) {
  + return colors.border[key] || key
  +}
  +
  const oppositeEdge = {
  top: 'Bottom',
  right: 'Left',
  @@ -71,7 +77,7 @@ export default function Caret(props) {
  <svg width={size * 2} height={size * 2} style={style}>
  <g transform={transform}>
  <path d={triangle} fill={fill} />
  - <path d={line} fill="none" stroke={borderColor} strokeWidth={borderWidth} />
  + <path d={line} fill="none" stroke={getBorderColor(borderColor)} strokeWidth={borderWidth} />
  </g>
  </svg>
  )
  @@ -93,7 +99,7 @@ Caret.locations = [
  ]
  Caret.defaultProps = {
  - borderColor: colors.gray[2],
  + borderColor: 'gray',
  borderWidth: 1,
  fill: colors.white,
  location: 'bottom',
  @@ -101,7 +107,7 @@ Caret.defaultProps = {
  }
  Caret.propTypes = {
  - borderColor: PropTypes.string,
  + borderColor: PropTypes.oneOf(borderColors),
  borderWidth: PropTypes.number,
  fill: PropTypes.string,
  location: PropTypes.oneOf(Caret.locations),
  