[runtime] Replace %to_string_fun with %_ToString.

src/harmony-regexp.js

 // Copyright 2014 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();
 
 // -------------------------------------------------------------------
 // Imports
 
 var GlobalRegExp = global.RegExp;
-var ToString;
-
-utils.Import(function(from) {
-  ToString = from.ToString;
-});
 
 // -------------------------------------------------------------------
 
 // ES6 draft 12-06-13, section 21.2.5.3
 // + https://bugs.ecmascript.org/show_bug.cgi?id=3423
 function RegExpGetFlags() {
   if (!IS_SPEC_OBJECT(this)) {
-    throw MakeTypeError(kFlagsGetterNonObject, ToString(this));
+    throw MakeTypeError(kFlagsGetterNonObject, TO_STRING(this));
   }
   var result = '';
   if (this.global) result += 'g';
   if (this.ignoreCase) result += 'i';
   if (this.multiline) result += 'm';
   if (this.unicode) result += 'u';
   if (this.sticky) result += 'y';
   return result;
 }
 
 %DefineAccessorPropertyUnchecked(GlobalRegExp.prototype, 'flags',
                                  RegExpGetFlags, null, DONT_ENUM);
 %SetNativeFlag(RegExpGetFlags);
 
 })