Use an "end of iteration" signal instead of undefined

The use of undefined as a sentinel for the end of iteration was problematic for value iterators, where the possible value space could contain undefined.

Author: domenic

index.bs

@@ -4354,13 +4354,13 @@ Prose accompanying an [=interface=] with an [=asynchronously iterable declaratio
 <dfn id="dfn-get-the-next-iteration-result" export>get the next iteration result</dfn> algorithm.
 This algorithm receives the instance of the [=interface=] that is being iterated, as well as the
 async iterator itself (which can be useful for storing state).
-It must return a {{Promise}} that either rejects, resolves with undefined to signal the end of the
-iteration, or resolves with one of the following:
+It must return a {{Promise}} that either rejects, resolves with a special <dfn export>end of
+iteration</dfn> value to signal the end of the iteration, or resolves with one of the following:
 
 : for [=value asynchronously iterable declarations=]:
 ::  a value of the type given in the declaration;
 : for [=pair asynchronously iterable declarations=]:
-::  a tuple containing a value of the first type given in the declaration, and a value of the second type given in the declaration.
+::  a [=pair=] containing a value of the first type given in the declaration, and a value of the second type given in the declaration.
 
 The prose may also define an <dfn export>asynchronous iterator return</dfn> algorithm. This
 algorithm receives the instance of the [=interface=] that is being iterated, the async iterator
@@ -4434,7 +4434,7 @@ or have any [=inherited interfaces=] that have [=interface members=] with these
             Note: |iterator|'s [=SessionManager async iterator/current state=] might no longer be
             present in the open sessions.
         1.  If |key| is null, then:
-            1.  Resolve |promise| with undefined.
+            1.  Resolve |promise| with [=end of iteration=].
         1.  Otherwise:
             1.  Let |session| be the <code class="idl">Session</code> object corresponding to |key|.
             1.  Resolve |promise| with (|username|, |session|).
@@ -12531,7 +12531,7 @@ The \[[Prototype]] [=internal slot=] of an [=asynchronous iterator prototype obj
         1.  Let |fulfillSteps| be the following steps, given |next|:
             1.  Set |object|'s [=default asynchronous iterator object/ongoing promise=] to
                 null.
-            1.  If |next| is <emu-val>undefined</emu-val>, then:
+            1.  If |next| is [=end of iteration=], then:
                 1.  Set |object|'s [=default asynchronous iterator object/is finished=] to true.
                 1.  Return [=!=] [$CreateIterResultObject$](<emu-val>undefined</emu-val>,
                     <emu-val>true</emu-val>).