message: remove textDecoration function, add prop types back
diff: -import React from 'react'
  import PropTypes from 'prop-types'
  import styled from 'react-emotion'
  -import {themeGet} from 'styled-system'
  import {withSystemProps, TYPOGRAPHY} from './system-props'
  import {colors} from './theme'
  const styledLink = styled('a')`
  - ${textDecoration};
  + text-decoration: ${props => props.underline ? 'underline' : 'none'};
  &:hover {
  text-decoration: underline;
  + color: ${colors.blue[5]};
  }
  `
  -function textDecoration({underline}) {
  - return {
  - textDecoration: underline ? 'underline' : 'none'
  - }
  +styledLink.propTypes = {
  + href: PropTypes.string,
  + underline: PropTypes.bool
  }
  -export default withSystemProps({
  +export default withSystemProps(
  + {
  is: styledLink,
  - color: 'blue.4'
  -}, TYPOGRAPHY)
  + color: 'blue.5'
  + },
  + TYPOGRAPHY
  +)
  