src/language-js/printer-estree.js/printPropertyKey
function printPropertyKey(path, options, print) {
  const node = path.getNode();
  const key = node.key;

  if (
    isStringLiteral(key) &&
    isIdentifierName(key.value) &&
    !node.computed &&
    options.parser !== "json"
  ) {
    // 'a' -> a
    return path.call(
      keyPath => comments.printComments(keyPath, () => key.value, options),
      "key"
    );
  }
  return path.call(print, "key");
}