code: function listRegionKeys() {
    const regionKeys = new Array(regionKeysByRegion.size);
    const iter = regionKeysByRegion.values();
    let v = iter.next();
    let i = -1;
    while (!v.done) {
      regionKeys[++i] = v.value;
      v = iter.next();
    }
    return regionKeys;
  }
docs: Lists the currently cached region keys (if any).
  @return {Array.<{region: string}>} a list of region keys