code: static Executor screenExecutor(Executor e) {
          if (!USE_COMMON_POOL && e == ForkJoinPool.commonPool())
              return ASYNC_POOL;
          if (e == null) throw new NullPointerException();
          return e;
      }
docs: Null-checks user executor argument, and translates uses of
  commonPool to ASYNC_POOL in case parallelism disabled.