Normative: specify web reality for Array.prototype.join with cyclic v…

…alues

Fixes tc39#289.

spec.html

@@ -5579,6 +5579,17 @@ <h1>Global Environment Records</h1>
                 Contains bindings for all declarations in global code for the associated realm code except for |FunctionDeclaration|, |GeneratorDeclaration|, |AsyncFunctionDeclaration|, |AsyncGeneratorDeclaration|, and |VariableDeclaration| _bindings_.
               </td>
             </tr>
+            <tr>
+              <td>
+                [[Seen]]
+              </td>
+              <td>
+                List of ECMAScript values
+              </td>
+              <td>
+                Exists as a realm-wide set of "seen" elements, to be used in, for example, Array.prototype.join
+              </td>
+            </tr>
             <tr>
               <td>
                 [[VarNames]]
@@ -32151,12 +32162,19 @@ <h1>Array.prototype.join ( _separator_ )</h1>
           1. Else, let _sep_ be ? ToString(_separator_).
           1. Let _R_ be the empty String.
           1. Let _k_ be 0.
+          1. Let _realm_ be the current Realm Record.
+          1. Let _seen_ be _realm_.[[Seen]].
           1. Repeat, while _k_ &lt; _len_
             1. If _k_ &gt; 0, set _R_ to the string-concatenation of _R_ and _sep_.
             1. Let _element_ be ? Get(_O_, ! ToString(_k_)).
-            1. If _element_ is *undefined* or *null*, let _next_ be the empty String; otherwise, let _next_ be ? ToString(_element_).
+            1. If _seen_ contains _element_, then
+              1. Let _next_ be the empty String.
+              1. Append _element_ to _seen_.
+            1. Else,
+              1. If _element_ is *undefined* or *null*, let _next_ be the empty String; otherwise, let _next_ be ? ToString(_element_).
             1. Set _R_ to the string-concatenation of _R_ and _next_.
             1. Increase _k_ by 1.
+          1. Set realm.[[Seen]] to a new empty List.
           1. Return _R_.
         </emu-alg>
         <emu-note>