Add Symbol to builtins

lib/propmangle.js

@@ -44,10 +44,16 @@
 "use strict";
 
 function find_builtins() {
+
+    // Compatibility fix for es5.1 and earlier where Symbol isn't defined
+    if (!global.Symbol) {
+        global.Symbol = new Function();
+    }
+
     var a = [];
     [ Object, Array, Function, Number,
       String, Boolean, Error, Math,
-      Date, RegExp
+      Date, RegExp, Symbol
     ].forEach(function(ctor){
         Object.getOwnPropertyNames(ctor).map(add);
         if (ctor.prototype) {