fixes #228: fail earlier when an iterable produces a broken iterator (#…

…232)

spec.html

@@ -214,6 +214,7 @@ <h1>Iterator.from ( _O_ )</h1>
             1. Let _usingIterator_ be ? GetMethod(_O_, @@iterator).
             1. If _usingIterator_ is not *undefined*, then
               1. Let _iteratorRecord_ be ? GetIterator(_O_, ~sync~, _usingIterator_).
+              1. If IsCallable(_iteratorRecord_.[[NextMethod]]) is *false*, throw a *TypeError* exception.
               1. Let _hasInstance_ be ? OrdinaryHasInstance(%Iterator%, _iteratorRecord_.[[Iterator]]).
               1. If _hasInstance_ is *true*, then
                 1. Return _iteratorRecord_.[[Iterator]].
@@ -296,13 +297,15 @@ <h1>AsyncIterator.from ( _O_ )</h1>
             1. Let _iteratorRecord_ be *undefined*.
             1. If _usingIterator_ is not *undefined*, then
               1. Set _iteratorRecord_ to ? GetIterator(_O_, ~async~, _usingIterator_).
+              1. If IsCallable(_iteratorRecord_.[[NextMethod]]) is *false*, throw a *TypeError* exception.
               1. Let _hasInstance_ be ? OrdinaryHasInstance(%AsyncIterator%, _iteratorRecord_.[[Iterator]]).
               1. If _hasInstance_ is *true*, then
                 1. Return _iteratorRecord_.[[Iterator]].
             1. If _iteratorRecord_ is *undefined*, then
               1. Set _usingIterator_ to ? GetMethod(_O_, @@iterator).
               1. If _usingIterator_ is not *undefined*, then
                 1. Let _syncIteratorRecord_ be ? GetIterator(_O_, ~sync~, _usingIterator_).
+                1. If IsCallable(_syncIteratorRecord_.[[NextMethod]]) is *false*, throw a *TypeError* exception.
                 1. Set _iteratorRecord_ to CreateAsyncFromSyncIterator(_syncIteratorRecord_).
             1. If _iteratorRecord_ is *undefined*, set _iteratorRecord_ to ? GetIteratorDirect(_O_).
             1. Let _wrapper_ be ! ObjectCreate(%WrapForValidAsyncIteratorPrototype%, &laquo; [[AsyncIterated]] &raquo;).