message: refactor(Symbol_iterator): remove warning
diff: export function getSymbolIterator(): symbol {
  - /* NOTE: Warning users that they don't have a Symbol.iterator
  - polyfill. We don't want to throw on this, because it's not required
  - by the library. However it will provide clues to users on older
  - browsers why things like `from(iterable)` doesn't work. */
  if (typeof Symbol !== 'function' || !Symbol.iterator) {
  - console.warn('RxJS: Symbol.iterator does not exist, so things like from(iterable) won\'t work');
  return '@@iterator' as any;
  }
  