Fixed #642, avoid “freezing” when calling CONS on non-list/null

thanks, @brianharvey !

history.txt

@@ -2321,3 +2321,7 @@ ______
 141117
 ------
 * Threads, Blocks: Treat REPORT blocks inside custom command definitions as STOP THIS BLOCK / IGNORE INPUTS
+
+141120
+------
+* Lists: Fixed #642, avoid “freezing” when calling CONS on non-list/null

lists.js

@@ -61,7 +61,7 @@ PushButtonMorph, SyntaxElementMorph, Color, Point, WatcherMorph,
 StringMorph, SpriteMorph, ScrollFrameMorph, CellMorph, ArrowMorph,
 MenuMorph, snapEquals, Morph, isNil, localize, MorphicPreferences*/
 
-modules.lists = '2014-July-28';
+modules.lists = '2014-November-20';
 
 var List;
 var ListWatcherMorph;
@@ -125,6 +125,9 @@ List.prototype.changed = function () {
 
 List.prototype.cons = function (car, cdr) {
     var answer = new List();
+    if (!(cdr instanceof List || isNil(cdr))) {
+        throw new Error("cdr isn't a list: " + cdr);
+    }
     answer.first = isNil(car) ? null : car;
     answer.rest = cdr || null;
     answer.isLinked = true;