go.tools/dashboard/builder: fix imported by not used error
go.tools/dashboard/builder: fix imported by not used error

Update golang/go#7372

This slipped into the build in revision 4b4229fc616b.

Thanks to @pierredurand87 for noticing the break.

LGTM=bgarcia
R=golang-codereviews, bgarcia
CC=golang-codereviews
https://golang.org/cl/66670043

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