Assertions for output of constructor description.

Fixes whatwg#206.

Author: tobie

index.bs

@@ -10363,17 +10363,12 @@ 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|.
+        |R| to an ECMAScript [=interface type=] value |I|.
 </ol>
 
-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|.
-
 Interface objects for non-callback interfaces must have a property named “length”
 with attributes <span class="descriptor">{ \[[Writable]]: <emu-val>false</emu-val>, \[[Enumerable]]: <emu-val>false</emu-val>, \[[Configurable]]: <emu-val>true</emu-val> }</span>
 whose value is a <emu-val>Number</emu-val>.