CAMBRIDGE - Chinese officials and private investors around the world have been worrying aloud about whether their dollar investments are safe. Since the Chinese government holds a large part of its $2 trillion of foreign exchange in dollars, they have good reason to focus on the future value of the greenback. And investors with smaller dollar holdings, who can shift to other currencies much more easily than the Chinese, are right to ask themselves whether they should be diversifying into non-dollar assets - or even shunning the dollar completely.