message: throw an error if withSystemProps() gets a system component
diff: @@ -38,9 +38,16 @@ export const FLEX_CONTAINER = LAYOUT.concat(
  export const FLEX_ITEM = LAYOUT.concat('justifySelf', 'alignSelf')
  +export function isSystemComponent(Component) {
  + return Component.systemComponent === true || Component.defaultProps && Array.isArray(Component.defaultProps.blacklist)
  +}
  +
  export function withSystemProps(Component, props = COMMON) {
  - const Wrapped = system({is: Component}, ...props)
  + if (isSystemComponent(Component)) {
  + throw new Error(`${Component.name} is already a system component; can't call withSystemProps() on it`)
  + }
  + const Wrapped = system({is: Component}, ...props)
  Object.assign(Wrapped.propTypes, Component.propTypes)
  // Copy over non-system keys from components
  @@ -50,6 +57,7 @@ export function withSystemProps(Component, props = COMMON) {
  Wrapped[key] = Component[key]
  }
  }
  +
  return withDefaultTheme(Wrapped)
  }
  