return new AsyncCallable<T>() {
      @Override
      public ListenableFuture<T> call() {
        return returnValue;
      }
    };
  }

  /** Runnable which can be called a single time, and only after {@link #expectCall} is called. */
  // TODO(cpovirk): top-level class?
  private static class SingleCallListener implements Runnable {

    private boolean expectCall = false;

    return new AsyncCallable<T>() {
      @Override
      public ListenableFuture<T> call() {
        return returnValue;
      }
    };
  }

  /** Runnable which can be called a single time, and only after {@link #expectCall} is called. */
  // TODO(cpovirk): top-level class?
  private static class SingleCallListener implements Runnable {

    private boolean expectCall = false;