build: Enable gometalinter "unused" check

build.go

@@ -299,6 +299,7 @@ func runCommand(cmd string, target target) {
 			ok = gometalinter("structcheck", dirs) && ok
 			ok = gometalinter("varcheck", dirs) && ok
 			ok = gometalinter("ineffassign", dirs) && ok
+			ok = gometalinter("unused", dirs) && ok
 			if !ok {
 				os.Exit(1)
 			}
@@ -369,6 +370,7 @@ func setup() {
 		"github.com/tsenart/deadcode",
 		"golang.org/x/net/html",
 		"golang.org/x/tools/cmd/cover",
+		"honnef.co/go/unused/cmd/unused",
 	}
 	for _, pkg := range packages {
 		fmt.Println(pkg)