function _shouldActuallyPolyfillES6Collection(collectionName: string): boolean {
  var Collection = global[collectionName];
  if (Collection == null) {
    return true;
  }

  // The iterator protocol depends on `Symbol.iterator`. If a collection is
  // implemented, but `Symbol` is not, it's going to break iteration because
  // we'll be using custom "@@iterator" instead, which is not implemented on
  // native collections.
  if (typeof global.Symbol !== 'function') {
    return true;
  }

  var proto = Collection.prototype;

  // These checks are adapted from es6-shim: https://fburl.com/34437854
  // NOTE: `isCallableWithoutNew` and `!supportsSubclassing` are not checked
  // because they make debugging with "break on exceptions" difficult.
  return Collection == null ||
    typeof Collection !== 'function' ||
    typeof proto.clear !== 'function' ||
    new Collection().size !== 0 ||
    typeof proto.keys !== 'function' ||
    typeof proto.forEach !== 'function';
}
Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
that is safe to be used.