条件编译不起作用

Question

OS: linux/amd64
GO: 1.5.1

When running (go run *.go) the below code I get the following error and I don't understand what I'm doing wrong. I've read the official doc about build constrains. Shouldn't foo_windows.go be ignored?

# command-line-arguments
./foo_windows.go:3: hello redeclared in this block
        previous declaration at ./foo_linux.go:3

main.go

package main

import "fmt"

func main() {
    fmt.Println(hello())
}

foo_linux.go

package main

func hello() string {
    return "hello form linux"
}

foo_windows.go

package main

func hello() string {
    return "hello form windows"
}