fixes #229: flatMap supports returning both iterables and iterators

spec.html

@@ -149,6 +149,37 @@ <h1>
         1. Return _iteratorRecord_.
       </emu-alg>
     </emu-clause>
+
+    <emu-clause id="sec-getiteratorflattenable" type="abstract operation">
+      <h1>
+        GetIteratorFlattenable (
+          _obj_: an ECMAScript language value,
+          optional _hint_: ~sync~ or ~async~,
+        ): either a normal completion containing an Iterator Record or a throw completion
+      </h1>
+      <dl class="header">
+      </dl>
+      <emu-alg>
+        1. If _hint_ is not present, set _hint_ to ~sync~.
+        1. If _iterator_ is either *undefined* or *null*, throw a *TypeError* exception.
+        1. Let _method_ be *undefined*.
+        1. If _hint_ is ~async~, then
+          1. Set _method_ to ? GetV(_obj_, @@asyncIterator).
+        1. If IsCallable(_method_) is *false*, then
+          1. Set _method_ to ? GetV(_obj_, @@iterator).
+        1. If IsCallable(_method_) is *false*, then
+          1. Let _iterator_ be _obj_.
+        1. Else,
+          1. Let _iterator_ be ? Call(_method_, _obj_).
+        1. If _iterator_ is not an Object, throw a *TypeError* exception.
+        1. Let _nextMethod_ be ? Get(_iterator_, *"next"*).
+        1. If IsCallable(_nextMethod_) is *false*, throw a *TypeError* exception.
+        1. Let _iteratorRecord_ be the Iterator Record { [[Iterator]]: _iterator_, [[NextMethod]]: _nextMethod_, [[Done]]: *false* }.
+        1. If _hint_ is ~async~, then
+          1. Return CreateAsyncFromSyncIterator(_iteratorRecord_).
+        1. Return _iteratorRecord_.
+      </emu-alg>
+    </emu-clause>
   </emu-clause>
 </emu-clause>
 
@@ -557,7 +588,7 @@ <h1>Iterator.prototype.flatMap ( _mapper_ )</h1>
               1. Let _value_ be ? IteratorValue(_next_).
               1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, &laquo; _value_ &raquo;)).
               1. IfAbruptCloseIterator(_mapped_, _iterated_).
-              1. Let _innerIterator_ be Completion(GetIterator(_mapped_, ~sync~)).
+              1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_, ~sync~)).
               1. IfAbruptCloseIterator(_innerIterator_, _iterated_).
               1. Let _innerAlive_ be *true*.
               1. Repeat, while _innerAlive_ is *true*,
@@ -823,7 +854,7 @@ <h1>AsyncIterator.prototype.flatMap ( _mapper_ )</h1>
               1. IfAbruptCloseAsyncIterator(_mapped_, _iterated_).
               1. Set _mapped_ to Completion(Await(_mapped_)).
               1. IfAbruptCloseAsyncIterator(_mapped_, _iterated_).
-              1. Let _innerIterator_ be Completion(GetIterator(_mapped_, ~async~)).
+              1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_, ~async~)).
               1. IfAbruptCloseAsyncIterator(_innerIterator_, _iterated_).
               1. Let _innerAlive_ be *true*.
               1. Repeat, while _innerAlive_ is *true*,