message: throw an error if we can't find the named system prop
diff: @@ -44,7 +44,8 @@ expect.extend({
  toImplementSystemProps(Component, propNames) {
  const propKeys = new Set(Object.keys(Component.propTypes))
  const expectedPropKeys = propNames.reduce((list, name) => {
  - const fn = systemProps[name]
  + const fn = (typeof name === 'function') ? name : systemProps[name]
  + if (!fn) throw new Error(`"${name}" is not a system prop!!`)
  return list.concat(Object.keys(fn.propTypes))
  }, [])
  const missing = expectedPropKeys.filter(key => !propKeys.has(key)).filter(key => !ALIAS_PROP_TYPES.includes(key))
  