function _cmdGetServer(path, port, cb) {
    // Make sure the key doesn't conflict with some built-in property of Object.
    var pathKey = getPathKey(path);
    if (_servers[pathKey]) {
        cb(null, _servers[pathKey].address());
    } else {
        _createServer(path, port, function (err, server) {
            if (err) {
                cb(err, null);
            } else {
                _servers[pathKey] = server;
                _rewritePaths[pathKey] = {};
                cb(null, server.address());
            }
        });
    }
}
@private
Handler function for the staticServer.getServer command. If a server
already exists for the given path, returns that, otherwise starts a new
one.
@param {string} path The absolute path that should be the document root
@param {function(?string, ?{address: string, family: string,
   port: number})} cb Callback that should receive the address information
   for the server. First argument is the error string (or null if no error),
   second argument is the address object (or null if there was an error).
   The "family" property of the address indicates whether the address is,
   for example, IPv4, IPv6, or a UNIX socket.