Also:
- always display the value of a constant expr, whether query expr is a
definition, a reference, or an alias.
- eliminate some go1.5 portability code.
- remove go1.8 fork of referrers; no changes are necessary
since I decided not to treat aliases specially.
- add tests.
Tested with Go 1.6, Go 1.7, and tip (Go 1.8).
Change-Id: I94624cff82f4d8c0dcbf12d11c8ce16e8168a7fe
Reviewed-on: https://go-review.googlesource.com/32730
Reviewed-by: Robert Griesemer <gri@golang.org>
d15da3015c