Normative: Relax [[DefineOwnProperty]] on module namespace exotic obj…

…ects.

[[DefineOwnProperty]] on module namespace exotic objects used to always
return false.  With this change, it returns true if, intuitively,
applying the given property descriptor wouldn't change anything.  This
is consistent with how [[DefineOwnProperty]] behaves for other kinds of
objects.

 - Special case for symbols.
 - Make OrdinaryDefineOwnProperty use IsExtensible.

spec.html

@@ -6861,7 +6861,7 @@ <h1>OrdinaryDefineOwnProperty ( _O_, _P_, _Desc_ )</h1>
         <p>When the abstract operation OrdinaryDefineOwnProperty is called with Object _O_, property key _P_, and Property Descriptor _Desc_, the following steps are taken:</p>
         <emu-alg>
           1. Let _current_ be ? _O_.[[GetOwnProperty]](_P_).
-          1. Let _extensible_ be _O_.[[Extensible]].
+          1. Let _extensible_ be ? IsExtensible(_O_).
           1. Return ValidateAndApplyPropertyDescriptor(_O_, _P_, _extensible_, _Desc_, _current_).
         </emu-alg>
       </emu-clause>
@@ -8488,7 +8488,15 @@ <h1>[[GetOwnProperty]] ( _P_ )</h1>
         <h1>[[DefineOwnProperty]] ( _P_, _Desc_ )</h1>
         <p>When the [[DefineOwnProperty]] internal method of a module namespace exotic object _O_ is called with property key _P_ and Property Descriptor _Desc_, the following steps are taken:</p>
         <emu-alg>
-          1. Return *false*.
+          1. If Type(_P_) is Symbol, return OrdinaryDefineOwnProperty(_O_, _P_, _Desc_).
+          1. Let _current_ be ? _O_.[[GetOwnProperty]](_P_).
+          1. If _current_ is *undefined*, return *false*.
+          1. If IsAccessorDescriptor(_Desc_) is *true*, return *false*.
+          1. If _Desc_.[[Writable]] is present and has value *false*, return *false*.
+          1. If _Desc_.[[Enumerable]] is present and has value *false*, return *false*.
+          1. If _Desc_.[[Configurable]] is present and has value *true*, return *false*.
+          1. If _Desc_.[[Value]] is present, return SameValue(_Desc_.[[Value]], _current_.[[Value]]).
+          1. Return *true*.
         </emu-alg>
       </emu-clause>