go.tools/cmd/vet: fix go.tools build TBR: gri
go.tools/cmd/vet: fix go.tools build
TBR: gri

I cannot create an issue on the tracker for some reason, so here it is:
go vet contains this snippet:

        if types.IsAssignableTo(typ, errorType) || types.IsAssignableTo(typ, stringerType) {

It's getting the wrong answer: It claims

interface {
        f()
}

or even

interface {
        f() float64
}

matches the Error and Stringer interfaces. Both of them.  This causes a test failure:
$ go test code.google.com/p/go.tools/cmd/vet
BUG: errchk: testdata/print.go:124: missing expected error: '"for printf verb %s of wrong type"'
$
This worked until very recently.

R=gri
CC=golang-dev
https://golang.org/cl/12398043

maddyonline/tools,argandas/tools,syreclabs/go-tools,cmars/tools,matryer/tools,glasser/golang-tools,golang/tools,josharian/tools,maddyonline/tools,nicot/tools,springning/tools,4ad/tools,runcom/tools,cw2018/tools,magiccao/tools,jdhenke/tools,nicot/tools,cw2018/tools,runcom/tools,nicot/tools,glasser/golang-tools,josharian/tools,josharian/tools,golang/tools,syreclabs/go-tools,sshetty10/tools,jdhenke/tools,syreclabs/go-tools,4ad/tools,magiccao/tools,jdhenke/tools,maddyonline/tools,matryer/tools,syreclabs/go-tools,nicot/tools,dmitris/tools,matryer/tools,dmitris/tools,springning/tools,runcom/tools,golang/tools,maddyonline/tools,dmitris/tools,cw2018/tools,glasser/golang-tools,cmars/tools,4ad/tools,cmars/tools,springning/tools,glasser/golang-tools,dmitris/tools,argandas/tools,sshetty10/tools,golang/tools,springning/tools,syreclabs/go-tools,argandas/tools,cmars/tools,jdhenke/tools,runcom/tools,josharian/tools,magiccao/tools,cw2018/tools,golang/tools,magiccao/tools,argandas/tools,matryer/tools,syreclabs/go-tools,sshetty10/tools,sshetty10/tools