getGlobalTokenPool_() {
    if (!EarthEngineTileSource.TOKEN_POOL_) {
      EarthEngineTileSource.TOKEN_POOL_ =
          new PriorityPool(0, EarthEngineTileSource.DEFAULT_TOKEN_COUNT_);
    }
    return EarthEngineTileSource.TOKEN_POOL_;
  }
@return {!PriorityPool} The global EE tile request token pool.
@private