src/language-js/printer-estree.js/getFlowVariance
function getFlowVariance(path) {
  if (!path.variance) {
    return null;
  }

  // Babylon 7.0 currently uses variance node type, and flow should
  // follow suit soon:
  // https://github.com/babel/babel/issues/4722
  const variance = path.variance.kind || path.variance;

  switch (variance) {
    case "plus":
      return "+";
    case "minus":
      return "-";
    default:
      /* istanbul ignore next */
      return variance;
  }
}