function _shouldPolyfillES6Collection(collectionName: string) {
    let result = cache[collectionName];
    if (result !== undefined) {
      return result;
    }

    result = _shouldActuallyPolyfillES6Collection(collectionName);
    cache[collectionName] = result;
    return result;
}
Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
that is safe to be used and caches this result.
Make sure to make a first call to this function before a corresponding
property on global was overriden in any way.