It doesn't work if the package name includes a '.' or a non-ASCII
character (or '%', '"', or a control character).  See #16710 and CL 31970.

Update #18246.

Change-Id: I1487f462a3dc7b0016fce3aa1ea6239b226e6e39
Reviewed-on: https://go-review.googlesource.com/34791Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>