Assertions for output of constructor description.

Fixes #206.

Author: tobie

index.bs

@@ -10316,14 +10316,11 @@ and |newTarget| as the NewTarget value, the following steps must be taken:
         [=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.  Assert: |R| is an object that implements |I|.
+    1.  Assert: |R|.\[[Realm]] is equal to |F|.\[[Realm]].
     1.  Return the result of [=converted to an ECMAScript value|converting=]
         |R| to an ECMAScript [=interface type=] value |I|.
 
-If evaluating the [=function object=] |F| returns normally,
-then it must return an object that implements interface |I|.
-This object also must be
-associated with the ECMAScript global environment associated
-with |I|.
 
 <div algorithm="determine value of length property of non-callback interfaces">