message: add test for withSystemProps() throwing
diff: @@ -12,6 +12,11 @@ describe('system props', () => {
  expect(Wrapped.propTypes).toEqual(fontFamily.propTypes)
  })
  + it('throws on attempting to wrap system components', () => {
  + const Component = withSystemProps('div', ['fontFamily'])
  + expect(() => withSystemProps(Component, ['space'])).toThrow()
  + })
  +
  it('renders the underlying component', () => {
  const Component = jest.fn(props => <b {...props} />)
  Component.propTypes = {
  