allow implementations to avoid creating String wrapper objects (#281)

spec.html

@@ -129,12 +129,14 @@ <h1>
       <h1>
         GetIteratorFlattenable (
           _obj_: an ECMAScript language value,
+          _stringHandling_: ~iterate-strings~ or ~reject-strings~,
         ): either a normal completion containing an Iterator Record or a throw completion
       </h1>
       <dl class="header">
       </dl>
       <emu-alg>
-        1. If _obj_ is not an Object, throw a *TypeError* exception.
+        1. If _obj_ is not an Object, then
+          1. If _stringHandling_ is ~reject-strings~ or _obj_ is not a String, throw a *TypeError* exception.
         1. Let _method_ be ? GetMethod(_obj_, @@iterator).
         1. If _method_ is *undefined*, then
           1. Let _iterator_ be _obj_.
@@ -187,8 +189,7 @@ <h1>Iterator.prototype</h1>
         <emu-clause id="sec-iterator.from">
           <h1>Iterator.from ( _O_ )</h1>
           <emu-alg>
-            1. If _O_ is a String, set _O_ to ! ToObject(_O_).
-            1. Let _iteratorRecord_ be ? GetIteratorFlattenable(_O_).
+            1. Let _iteratorRecord_ be ? GetIteratorFlattenable(_O_, ~iterate-strings~).
             1. Let _hasInstance_ be ? OrdinaryHasInstance(%Iterator%, _iteratorRecord_.[[Iterator]]).
             1. If _hasInstance_ is *true*, then
               1. Return _iteratorRecord_.[[Iterator]].
@@ -415,7 +416,7 @@ <h1>Iterator.prototype.flatMap ( _mapper_ )</h1>
               1. Let _value_ be ? IteratorValue(_next_).
               1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
               1. IfAbruptCloseIterator(_mapped_, _iterated_).
-              1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_)).
+              1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_, ~reject-strings~)).
               1. IfAbruptCloseIterator(_innerIterator_, _iterated_).
               1. Let _innerAlive_ be *true*.
               1. Repeat, while _innerAlive_ is *true*,