Clarify object creation in named constructors (#652)

This matches the text for normal constructors. I forgot to update this
algorithm in 8e1b529.

index.bs

@@ -10724,11 +10724,10 @@ the <code>typeof</code> operator will return "function" when applied to an inter
         1.  Perform the actions listed in the description of |constructor|
             with |values| as the argument values
             and |object| as the <emu-val>this</emu-val> value.
-            Rethrow any exceptions.
         1.  Let |O| be the result of [=converted to an ECMAScript value|converting=] |object|
             to an ECMAScript [=interface type=] value |I|.
         1.  Assert: |O| is an object that implements |I|.
-        1.  Assert: |O|.\[[Realm]] is equal to |realm|.
+        1.  Assert: |O|.\[[Realm]] is |realm|.
         1.  Return |O|.
     1.  Let |constructorProto| be |realm|.\[[Intrinsics]].[[{{%FunctionPrototype%}}]].
     1.  If |I| inherits from some other interface |P|,
@@ -10763,10 +10762,6 @@ It allows constructing objects that
 implement the interface on which the
 [{{NamedConstructor}}] extended attributes appear.
 
-If the actions listed in the description of the constructor return normally,
-then those steps must return an object that implements interface |I|.
-This object's relevant [=Realm=] must be the same as that of the [=named constructor=].
-
 <div algorithm>
 
     The [=named constructor=] with [=NamedConstructor identifier|identifier=] |id|
@@ -10783,10 +10778,17 @@ This object's relevant [=Realm=] must be the same as that of the [=named constru
             and with argument count |n|.
         1.  Let &lt;|constructor|, |values|&gt; be the result of passing |S| and
             |args| to the [=overload resolution algorithm=].
-        1.  Let |R| be the result of performing the actions listed in the description of
-            |constructor| with |values| as the argument values.
-        1.  Return the result of [=converted to an ECMAScript value|converting=]
-            |R| to an ECMAScript [=interface type=] value |I|.
+        1.  Let |object| be the result of [=internally create a new object implementing the
+            interface|internally creating a new object implementing=] |I|, with |realm| and
+            {{NewTarget}}.
+        1.  Perform the actions listed in the description of |constructor|
+            with |values| as the argument values
+            and |object| as the <emu-val>this</emu-val> value.
+        1.  Let |O| be the result of [=converted to an ECMAScript value|converting=] |object|
+            to an ECMAScript [=interface type=] value |I|.
+        1.  Assert: |O| is an object that implements |I|.
+        1.  Assert: |O|.\[[Realm]] is |realm|.
+        1.  Return |O|.
     1.  Let |F| be [=!=] <a abstract-op>CreateBuiltinFunction</a>(|steps|, « », |realm|).
     1.  Perform [=!=] <a abstract-op>SetFunctionName</a>(|F|, |id|).
     1.  Initialize |S| to the [=effective overload set=]