*
   * ```java
   * assertEquals("google.com", getEffectiveTldPlusOne("google.com"));
   * assertEquals("google.com", getEffectiveTldPlusOne("www.google.com"));
   * assertNull(getEffectiveTldPlusOne("com"));
   * assertNull(getEffectiveTldPlusOne("localhost"));
   * assertNull(getEffectiveTldPlusOne("mymacbook"));
   * ```
   *
   * @param domain A canonicalized domain. An International Domain Name (IDN) should be punycode
   *     encoded.