With the (partial) resolution of identifiers done
by the go/parser, ast.Objects point may introduce
cycles in the AST. Don't follow *ast.Objects, and
replace them with nil instead (they are likely
incorrect after a rewrite anyway).

- minor manual cleanups after reflect change automatic rewrite
- includes fix by rsc related to reflect change

Fixes #1667.

R=rsc
CC=golang-dev
https://golang.org/cl/4387044