cmd/compile: unnecessary error after "cannot refer to unexported type"

[mvdan]

https://play.golang.org/p/ZwFSxT9JCu

Output on both 1.9.2 and tip:

tmp/sandbox627872070/main.go:6:8: cannot refer to unexported name bytes.buffer
tmp/sandbox627872070/main.go:6:8: undefined: bytes.buffer

Is the second error really necessary? It seems redundant to me, unless there is a good reason for it to be there.

/cc @mdempsky @griesemer

[griesemer]

There's no need for two errors.

[mvdan]

Doesn't seem important enough to keep around for 1.12. Also, should @mdempsky still be assigned?