Normative: add Iterator.concat

spec.html

@@ -48086,6 +48086,36 @@ <h1>Properties of the Iterator Constructor</h1>
           <li>has the following properties:</li>
         </ul>
 
+        <emu-clause id="sec-iterator.concat">
+          <h1>Iterator.concat ( ..._items_ )</h1>
+          <emu-alg>
+            1. Let _iterables_ be a new empty List.
+            1. For each element _item_ of _items_, do
+              1. If _item_ is not an Object, throw a *TypeError* exception.
+              1. Let _method_ be ? GetMethod(_item_, %Symbol.iterator%).
+              1. If _method_ is *undefined*, throw a *TypeError* exception.
+              1. Append the Record { [[OpenMethod]]: _method_, [[Iterable]]: _item_ } to _iterables_.
+            1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterables_ and performs the following steps when called:
+              1. For each Record _iterable_ of _iterables_, do
+                1. Let _iter_ be ? Call(_iterable_.[[OpenMethod]], _iterable_.[[Iterable]]).
+                1. If _iter_ is not an Object, throw a *TypeError* exception.
+                1. Let _iteratorRecord_ be ? GetIteratorDirect(_iter_).
+                1. Let _innerAlive_ be *true*.
+                1. Repeat, while _innerAlive_ is *true*,
+                  1. Let _innerValue_ be ? IteratorStepValue(_iteratorRecord_).
+                  1. If _innerValue_ is ~done~, then
+                    1. Set _innerAlive_ to *false*.
+                  1. Else,
+                    1. Let _completion_ be Completion(Yield(_innerValue_)).
+                    1. If _completion_ is an abrupt completion, then
+                      1. Return ? IteratorClose(_iteratorRecord_, _completion_).
+              1. Return ReturnCompletion(*undefined*).
+            1. Let _gen_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterators]] »).
+            1. Set _gen_.[[UnderlyingIterators]] to a new empty List.
+            1. Return _gen_.
+          </emu-alg>
+        </emu-clause>
+
         <emu-clause id="sec-iterator.from">
           <h1>Iterator.from ( _O_ )</h1>
           <emu-alg>