It is insufficiently canonical;
see the discussion at issue 19719.

Fixes #19719

Change-Id: I0559ff3b1b39d7bc4b446d104f36fdf8ce3da50e
Reviewed-on: https://go-review.googlesource.com/38722
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>