fix mangling

(bug in our code prevented the mangler from using a name that was in use
prior to mangling but not after it)

lib/scope.js

@@ -291,7 +291,7 @@ AST_Scope.DEFMETHOD("next_mangled", function(){
         // inner scopes.
         for (var i = n; --i >= 0;) {
             var sym = ext[i];
-            var name = sym.mangled_name || sym.name;
+            var name = sym.mangled_name || (sym.unmangleable() && sym.name);
             if (m == name) continue out;
         }
         return m;