[release-branch.r57] reflect: allow unexported key in Value.MapIndex

««« CL 4444087 / 9abf81a9df90
reflect: allow unexported key in Value.MapIndex

Fixes #1748.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4444087
»»»

R=r, r2
CC=golang-dev
https://golang.org/cl/4433099