We use -1 for Phi argLength, not "a really large number".

Change-Id: I5d4fa364c22c3cf40213f2368a9a20de7e989aac
Reviewed-on: https://go-review.googlesource.com/c/go/+/203883
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>