function $reactive (received, expected) {
  const pass = (
    isReactive(received)
    && (expected === void 0 || this.equals(received, expected))
  )

  return {
    pass,
    message: () =>
      `expected ${ this.utils.printReceived(
        received
      ) } to${ this.isNot ? ' not' : '' } be reactive${ expected !== void 0
        ? ` and be equal to ${ this.utils.printExpected(expected) }`
        : ''
      }`
  }
}
Examples:
  expect(variable).$reactive()
  expect.$reactive()
  expect.$reactive(<someValue>)
  expect.$reactive(expect.any(Boolean))