Enable ES6 Symbols by default

src/d8.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 "use strict";
 
 String.prototype.startsWith = function (str) {
   if (str.length > this.length) {
     return false;
   }
@@ skipping 1979 matching lines @@
             {}.hasOwnProperty.call(x, i) ? Stringify(x[i], depth - 1) : "");
         }
         return "[" + elems.join(", ") + "]";
       }
       try {
         var string = String(x);
         if (string && string !== "[object Object]") return string;
       } catch(e) {}
       var props = [];
       var names = Object.getOwnPropertyNames(x);
-      if (Object.getOwnPropertySymbols) {
-        // FLAG_harmony_symbols is turned on.
-        names = names.concat(Object.getOwnPropertySymbols(x));
-      }
+      names = names.concat(Object.getOwnPropertySymbols(x));
       for (var i in names) {
         var name = names[i];
         var desc = Object.getOwnPropertyDescriptor(x, name);
         if (IS_UNDEFINED(desc)) continue;
         if (IS_SYMBOL(name)) name = "[" + Stringify(name) + "]";
         if ("value" in desc) {
           props.push(name + ": " + Stringify(desc.value, depth - 1));
         }
         if (desc.get) {
           var getter = Stringify(desc.get);
           props.push("get " + name + getter.slice(getter.indexOf('(')));
         }
         if (desc.set) {
           var setter = Stringify(desc.set);
           props.push("set " + name + setter.slice(setter.indexOf('(')));
         }
       }
       return "{" + props.join(", ") + "}";
     default:
       return "[crazy non-standard shit]";
   }
 }