go/ast/astutil: do not remove imports in AddNamedImport

When adding imports to an existing import declaration, ensure that
the target declaration will be printed with parentheses. This allows
all of the imported packages to be printed, not just the first one.

Fixes golang/go#17212

Change-Id: Ie5de5ec9bca6169650336ee2ea98334817e64e48
Reviewed-on: https://go-review.googlesource.com/29688
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed