code: public final FluentFuture<V> catching(
        Class<Throwable> exceptionType,
        Function<? super Throwable, ? extends V> fallback,
        Executor executor) {
      return (FluentFuture<V>) Futures.catching(this, exceptionType, fallback, executor);
    }
docs: /*
  In the GWT versions of the methods (below), every exceptionType parameter is required to be
  Class<Throwable>. To handle only certain kinds of exceptions under GWT, you'll need to write
  your own instanceof tests.