Importing fully-qualified "appengine" package causes unsafe/syscall errors

Vagrantfile

# -*- mode: ruby -*-
# vi: set ft=ruby :

# Vagrantfile API/syntax version. Don't touch unless you know what you're doing!
VAGRANTFILE_API_VERSION = "2"

$script = <<SCRIPT
apt-get update
apt-get install -y unzip git
wget -nv https://storage.googleapis.com/appengine-sdks/featured/go_appengine_sdk_linux_amd64-1.9.40.zip
unzip go_appengine_sdk_linux_amd64-1.9.40.zip 

mkdir testproject
cd testproject

cat << EOF > app.yaml
application: testapp
version: 1
runtime: go
api_version: go1

handlers:
- url: /.*
  script: _go_app
EOF

cat << EOF > main.go
package testapp

import (
    "log"

    "google.golang.org/appengine"
//    "appengine"
)

func init() {
    log.Print(appengine.IsDevAppServer())
}

func main() {
    // This is only here because go-get needs on it.
}
EOF

GOPATH=`pwd` ../go_appengine/goapp get
#GOPATH=`pwd` ../go_appengine/goapp serve
SCRIPT

Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
  config.vm.box = "ubuntu/trusty64"
  config.vm.provision "shell", inline: $script
end

# Run "GOPATH=`pwd` ../go_appengine/goapp serve" inside.
#
# Produces error (may not produce the same one every time):
#
# 2016/08/02 15:20:28 go-app-builder: Failed parsing input: parser: bad import "syscall" in src/golang.org/x/net/ipv4/dgramopt_posix.go