Always allocate symbols in old space.

test/mjsunit/harmony/symbols.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 283 matching lines @@
   var obj = objs[i]
   TestKeySet(obj)
   TestKeyDefine(obj)
   TestKeyGet(obj)
   TestKeyHas(obj)
   TestKeyEnum(obj)
   TestKeyNames(obj)
   TestKeyDescriptor(obj)
   TestKeyDelete(obj)
 }
+
+
+function TestCachedKeyAfterScavenge() {
+  gc();
+  // Keyed property lookup are cached.  Hereby we assume that the keys are
+  // tenured, so that we only have to clear the cache between mark compacts,
+  // but not between scavenges.  This must also apply for symbol keys.
+  var key = Symbol("key");
+  var a = {};
+  a[key] = "abc";
+  
+  for (var i = 0; i < 1000000; i++) {
+    a[key] += "a";  // Allocations cause a scavenge.
+  }
+}
+TestCachedKeyAfterScavenge();