Import "golang.org/x/net/icmp" Failed!

I'm new to Golang and I cant find any good resource about this subject

I see that people use import "golang.org/x/net/icmp" or things like this(https://github.com/tatsushid/go-fastping/blob/master/fastping.go),but I cant get them work ! 
I tried to install the package but get error like

go get golang.org/x/net 
package golang.org/x/net: unrecognized import path "golang.org/x/net"

or
go get github.com/golang/net

but all the import are pointing to golang.org/x/net/...

I don't know how should I use packages golang.org/x/net/icmp  and other related ones...

解决方法：