Don't make async-from-sync iterator objects observable from user code

Fixes #1172

spec.html

@@ -3355,7 +3355,7 @@ <h1>CompletePropertyDescriptor ( _Desc_ )</h1>
  <p>When the abstract operation CompletePropertyDescriptor is called with Property Descriptor _Desc_, the following steps are taken:</p>
  <emu-alg>
  1. Assert: _Desc_ is a Property Descriptor.
- 1. Let _like_ be Record { [[Value]]: *undefined*, [[Writable]]: *false*, [[Get]]: *undefined*, [[Set]]: *undefined*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.
+ 1. Let _like_ be the Record { [[Value]]: *undefined*, [[Writable]]: *false*, [[Get]]: *undefined*, [[Set]]: *undefined*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.
  1. If IsGenericDescriptor(_Desc_) is *true* or IsDataDescriptor(_Desc_) is *true*, then
  1. If _Desc_ does not have a [[Value]] field, set _Desc_.[[Value]] to _like_.[[Value]].
  1. If _Desc_ does not have a [[Writable]] field, set _Desc_.[[Writable]] to _like_.[[Writable]].
@@ -4869,7 +4869,7 @@ <h1>GetIterator ( _obj_ [ , _hint_ [ , _method_ ] ] )</h1>
  1. Let _iterator_ be ? Call(_method_, _obj_).
  1. If Type(_iterator_) is not Object, throw a *TypeError* exception.
  1. Let _nextMethod_ be ? GetV(_iterator_, `"next"`).
- 1. Let _iteratorRecord_ be Record { [[Iterator]]: _iterator_, [[NextMethod]]: _nextMethod_, [[Done]]: *false* }.
+ 1. Let _iteratorRecord_ be the Record { [[Iterator]]: _iterator_, [[NextMethod]]: _nextMethod_, [[Done]]: *false* }.
  1. Return _iteratorRecord_.
  </emu-alg>
  </emu-clause>
@@ -36845,7 +36845,9 @@ <h1>CreateAsyncFromSyncIterator ( _syncIteratorRecord_ )</h1>
  <emu-alg>
  1. Let _asyncIterator_ be ! ObjectCreate(%AsyncFromSyncIteratorPrototype%, &laquo; [[SyncIteratorRecord]] &raquo;).
  1. Set _asyncIterator_.[[SyncIteratorRecord]] to _syncIteratorRecord_.
- 1. Return ? GetIterator(_asyncIterator_, ~async~).
+ 1. Let _nextMethod_ be ! Get(_asyncIterator_, `"next"`).
+ 1. Let _iteratorRecord_ be the Record { [[Iterator]]: _asyncIterator_, [[NextMethod]]: _nextMethod_, [[Done]]: *false* }.
+ 1. Return _iteratorRecord_.
  </emu-alg>
  </emu-clause>