message: don't concat categories in system-props
diff: @@ -5,18 +5,20 @@ import {default as defaultTheme} from './theme'
  export {system as default}
  +export const printProps = (props) => props.join(', ')
  +
  export const COMMON = ['color', 'space']
  -export const TYPOGRAPHY = COMMON.concat(
  +export const TYPOGRAPHY = [
  // typography props
  'fontFamily',
  'fontSize',
  'fontWeight',
  'lineHeight',
  'textAlign'
  -)
  +]
  -export const LAYOUT = COMMON.concat(
  +export const LAYOUT = [
  // layout props
  'display',
  'size',
  @@ -28,7 +30,7 @@ export const LAYOUT = COMMON.concat(
  'maxHeight',
  'overflow',
  'verticalAlign'
  -)
  +]
  export const POSITION = [
  // position props
  @@ -40,9 +42,8 @@ export const POSITION = [
  'left'
  ]
  -export const FLEX_CONTAINER = LAYOUT.concat(
  +export const FLEX_CONTAINER = [
  // flex container props (display: flex)
  - 'flex',
  'flexBasis',
  'flexDirection',
  'flexWrap',
  @@ -51,13 +52,14 @@ export const FLEX_CONTAINER = LAYOUT.concat(
  'justifyContent',
  'justifyItems',
  'order'
  -)
  +]
  -export const FLEX_ITEM = LAYOUT.concat(
  +export const FLEX_ITEM = [
  // flex container child props
  + 'flex',
  'justifySelf',
  'alignSelf'
  -)
  +]
  /**
  * Defensively determine whether a component function or class is a "system
  