$ pwd
/tmp
$ mkdir go go2
$ export GOPATH=/tmp/go
$ go get github.com/gopherjs/gopherjs
$ export PATH=/tmp/go/bin:$PATH
$ cd go2
$ mkdir -p src/testpackage/
$ cat src/testpackage/main.go 
package main

func main() {

println("Hello, world!")

}

$ export GOPATH=/tmp/go2
$ go build testpackage
$ ./testpackage 
Hello, world!
$ gopherjs build testpackage
../../usr/lib/go/src/runtime/error.go:58:7: undeclared name: efaceOf