They are impeaching a President of the United States. Any whistleblower statutes are designed to protect the whistleblower from reprisals. It does not shield him from scrutiny. Let him come forward. Once he has then we will know from that day forward if he was fired or threatened or whatever.
Donald Trump should be able to face his accuser, and impeach (no pun intended) his credibility. Is there such a thing as a Stalinist kangaroo?