message: Allow FastPath#get{Prev,Next}Token to be called with no argument.
diff: @@ -285,6 +285,7 @@ FPp.hasParens = function () {
  };
  FPp.getPrevToken = function (node) {
  + node = node || this.getNode();
  const loc = node && node.loc;
  const tokens = loc && loc.tokens;
  if (tokens && loc.start.token > 0) {
  @@ -301,6 +302,7 @@ FPp.getPrevToken = function (node) {
  };
  FPp.getNextToken = function (node) {
  + node = node || this.getNode();
  const loc = node && node.loc;
  const tokens = loc && loc.tokens;
  if (tokens && loc.end.token < tokens.length) {
  