go run: correctly handle -n and -x flags

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5557069

src/cmd/go/run.go

@@ -4,7 +4,9 @@
 
 package main
 
-import ()
+import (
+	"strings"
+)
 
 var cmdRun = &Command{
 	UsageLine: "run [-a] [-n] [-x] gofiles... [-- arguments...]",
@@ -42,6 +44,12 @@ func runRun(cmd *Command, args []string) {
 // runProgram is the action for running a binary that has already
 // been compiled.  We ignore exit status.
 func (b *builder) runProgram(a *action) error {
+	if buildN || buildX {
+		b.showcmd("", "%s %s", a.deps[0].target, strings.Join(a.args, " "))
+		if buildN {
+			return nil
+		}
+	}
 	run(a.deps[0].target, a.args)
 	return nil
 }