The I' is not absolutely necessary, but I think Shakespeare wrote it. See on Hamlet, iii. 1.
Sc. 4.
"Is execution done on Cawdor? or not
Those in commission yet returned?"
There is every reason to suppose that the poet wrote 'are not,' the reading of the 2nd folio.
Sc. 5.
"That which cries "Thus must thou do, if thou'dst have it."