message: export breakpoints from props.js
diff: import PropTypes from 'prop-types'
  import createMapper from 'system-classnames'
  -const breakpoints = [null, 'sm', 'md', 'lg', 'xl']
  +export const breakpoints = [null, 'sm', 'md', 'lg', 'xl']
  export const oneOrMoreOf = type => PropTypes.oneOfType([type, PropTypes.arrayOf(type)])
  