ES6 symbols: turn symbols into a proper primitive type

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 934c82eb10566973cd6fead748b1144f10b20989..4908774aeceb4ec3704a5be1e1a1a791b51f4c85 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -5035,6 +5035,8 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_Typeof) {
       }
       ASSERT(heap_obj->IsUndefined());
       return isolate->heap()->undefined_string();
+    case SYMBOL_TYPE:
+      return isolate->heap()->symbol_string();
     case JS_FUNCTION_TYPE:
     case JS_FUNCTION_PROXY_TYPE:
       return isolate->heap()->function_string();