% echo $GOPATH
/home/mpl

% pwd
/home/mpl/src/foo

% cat baz/main.go 
package main

import (
    "foo/echo"
)

func main() {
    echo.Echo()
}

% cat echo/echo.go 
package echo

import "bar"

func Echo() {
    println(bar.Hello)
}

% cat vendor/bar/bar.go 
package bar

const Hello = "hello world"

% cd baz
% go build
% ./baz
hello world

% gopherjs build
cannot find package "bar" in any of:
    /home/mpl/go1/src/bar (from $GOROOT)
    /home/mpl/src/bar (from $GOPATH)

% go version
go version go1.6 linux/amd64

gopherjs built at rev 14ba2f52062a79512bbb6768908c95032395ce94