go/token: add test for concurrent use of FileSet.Pos
go/token: add test for concurrent use of FileSet.Pos

Update issue 4354.

Add a test to expose the race in the FileSet position cache.

R=dvyukov, gri
CC=fullung, golang-dev
https://codereview.appspot.com/6940078

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme