String closingFutureToString() {
      // TODO(dpb): Better toString, in the style of Futures.transform etc.
      return toStringHelper("ClosingFuture")
          .add("status", status.get())
          .addValue(future)
          .toString();
    }
  }

  // TODO(dpb): Should we use a pair of ArrayLists instead of an IdentityHashMap?