A witness? He is part of the conspiracy to take down Trump.
If it was the other way around, how fast do you think Chuck Schumer would want that FBI informant identified?!!
Look at what I said in my last post, and understand it in the context of Sen. Grassley's letter to Rosenstein two days ago where he demands to see an unredacted copy of Rosenstein's memo outlining Mueller's responsibilities. The source is named in that document. Grassley is asking to see something that has already been given under seal to a Federal judge presiding over a criminal trial.
As I said months ago, these trials are the best way to get this information out in the open because criminal defendants can demand to see information related to their cases that would otherwise be kept out of public view.