mac 执行gofmt -w test.go 无法格式化代码

``` GOARCH="amd64" GOBIN="/Users/xiaojiong/go/bin" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOOS="darwin" GOPATH="/Users/xiaojiong/go" GORACE="" GOROOT="/usr/local/go" GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64" GO15VENDOREXPERIMENT="" CC="clang" GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fno-common" CXX="clang++" CGO_ENABLED="1" `` test.go如下： package main import "fmt" func main() { //这行我故意放在这的 fmt.Printf("hello, world\n") }