_getMapKey(nodes) {
    var key = nodes[0].type;
    var length = nodes.length;

    // Significantly faster than a map & join
    for (var i = 1; i < length; i++) {
      key += ':' + nodes[i].type;
    }

    // Prefer shorter key lengths (base64 < hex)
    return crypto.createHash('sha1').update(key).digest('base64');
  }
Generates a key based on the type of each of the passed nodes, returned
as a base64-encoded sha1 hash.

@private

@param   {Node[]} nodes The nodes for which to generate the key
@returns {string}