code: function warnMissedAreas (areas, decl, result) {
    let missed = Object.keys(areas)
  
    decl.root().walkDecls('grid-area', gridArea => {
      missed = missed.filter(e => e !== gridArea.value)
    })
  
    if (missed.length > 0) {
      decl.warn(result, 'Can not find grid areas: ' + missed.join(', '))
    }
  
    return undefined
  }
docs: Warn user if grid area identifiers are not found
  @param  {Object} areas
  @param  {Declaration} decl
  @param  {Result} result
  @return {void}