split up the steps and add an assert for better understandability

spec.html

@@ -7007,7 +7007,9 @@ <h1>
       </dl>
       <emu-alg>
         1. If _obj_ is not an Object, then
-          1. If _stringHandling_ is ~reject-primitives~ or _obj_ is not a String, throw a *TypeError* exception.
+          1. If _primitiveHandling_ is ~reject-primitives~, throw a *TypeError* exception.
+          Assert: _primitiveHandling_ is ~iterate-string-primitives~.
+          1. If _obj_ is not a String, throw a *TypeError* exception.
         1. Let _method_ be ? GetMethod(_obj_, %Symbol.iterator%).
         1. If _method_ is *undefined*, then
           1. Let _iterator_ be _obj_.