Move builtin JavaScript sources into own directory.

src/harmony-tostring.js

Index: src/harmony-tostring.js
diff --git a/src/harmony-tostring.js b/src/harmony-tostring.js
deleted file mode 100644
index 8e76c3a5bbd97cdece907673070a6a44441ed806..0000000000000000000000000000000000000000
--- a/src/harmony-tostring.js
+++ /dev/null
@@ -1,19 +0,0 @@
-// Copyright 2013 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.
-
-(function(global, utils) {
-
-"use strict";
-
-%CheckIsBootstrapping();
-
-var GlobalSymbol = global.Symbol;
-var toStringTagSymbol = utils.ImportNow("to_string_tag_symbol");
-
-utils.InstallConstants(GlobalSymbol, [
-   // TODO(dslomov, caitp): Move to symbol.js when shipping
-   "toStringTag", toStringTagSymbol
-]);
-
-})