code: public static JSTypeExpression makeOptionalArg(JSTypeExpression expr) {
      if (expr.isOptionalArg() || expr.isVarArgs()) {
        return expr;
      } else {
        return new JSTypeExpression(
            new Node(Token.EQUALS, expr.root), expr.sourceName);
      }
    }
docs: Make the given type expression into an optional type expression,
  if possible.