Fix issue by always appending newline after user input, before
the closing curly bracket. The adjust func is modified to remove
this new newline.

Add test case (it fails before CL, passes after).

Fixes #8411.

LGTM=gri
R=golang-codereviews, bradfitz, josharian, gri
CC=golang-codereviews
https://golang.org/cl/124700043