Normative: Verify if Binding is preserved for obj's SetMutableBinding (…

…tc39#2094)

Verify binding existence in the object's SetMutableBinding on Strict Mode.

Closes tc39#2093
Ref tc39#467
Ref tc39/test262#427

spec.html

@@ -6626,6 +6626,8 @@ <h1>SetMutableBinding ( _N_, _V_, _S_ )</h1>
           <emu-alg>
             1. Let _envRec_ be the object Environment Record for which the method was invoked.
             1. Let _bindings_ be the binding object for _envRec_.
+            1. Let _stillExists_ be ? HasProperty(_bindings_, _N_).
+            1. If _stillExists_ is *false* and _S_ is *true*, throw a *ReferenceError* exception.
             1. Return ? Set(_bindings_, _N_, _V_, _S_).
           </emu-alg>
         </emu-clause>