This disrupts code instrumentation tools like go-fuzz.

Change-Id: I524f31316975096ca5e3b1203a82e91ed1b6097e
Reviewed-on: https://go-review.googlesource.com/c/go/+/168801
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>