net: fix the build even harder

Should fix nacl.

Follow-up to
https://golang.org/cl/17936 (fix race) and
https://golang.org/cl/17914 (fix build) for
https://golang.org/cl/16953 (broke the build)

Third time's a charm.

Change-Id: I23930d5cff4235209546952ce2231f165ab5bf8a
Reviewed-on: https://go-review.googlesource.com/17939
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/dnsclient_unix.go

@@ -29,6 +29,8 @@ type dnsDialer interface {
 	dialDNS(string, string) (dnsConn, error)
 }
 
+var testHookDNSDialer = func(d time.Duration) dnsDialer { return &Dialer{Timeout: d} }
+
 // A dnsConn represents a DNS transport endpoint.
 type dnsConn interface {
 	io.Closer

src/net/hook_unix.go

@@ -6,14 +6,10 @@
 
 package net
 
-import (
-	"syscall"
-	"time"
-)
+import "syscall"
 
 var (
 	testHookDialChannel = func() {} // see golang.org/issue/5349
-	testHookDNSDialer   = func(d time.Duration) dnsDialer { return &Dialer{Timeout: d} }
 
 	// Placeholders for socket system calls.
 	socketFunc        func(int, int, int) (int, error)         = syscall.Socket