docs: public static <T> T getCompleted(CompletionStage<T> stage) {
      CompletableFuture<T> future = stage.toCompletableFuture();
      Preconditions.checkArgument(future.isDone() && !future.isCompletedExceptionally());
      try {
        return future.get();
      } catch (InterruptedException | ExecutionException e) {
        // Neither can happen given the precondition
        throw new AssertionError("Unexpected error", e);
      }
    }