[regexp] implement RegExp.prototype.toString for non-RegExp receiver.

src/js/regexp.js

 // Copyright 2012 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) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
@@ skipping 260 matching lines @@
 
 function RegExpToString() {
   if (!IS_REGEXP(this)) {
     // RegExp.prototype.toString() returns '/(?:)/' as a compatibility fix;
     // a UseCounter is incremented to track it.
     // TODO(littledan): Remove this workaround or standardize it
     if (this === GlobalRegExpPrototype) {
       %IncrementUseCounter(kRegExpPrototypeToString);
       return '/(?:)/';
     }
-    throw MakeTypeError(kIncompatibleMethodReceiver,
-                        'RegExp.prototype.toString', this);
+    if (!IS_RECEIVER(this)) {
+      throw MakeTypeError(
+          kIncompatibleMethodReceiver, 'RegExp.prototype.toString', this);
+    }
+    return '/' + TO_STRING(this.pattern()) + '/' + TO_STRING(this.flags());
   }
   var result = '/' + REGEXP_SOURCE(this) + '/';
   if (REGEXP_GLOBAL(this)) result += 'g';
   if (REGEXP_IGNORE_CASE(this)) result += 'i';
   if (REGEXP_MULTILINE(this)) result += 'm';
   if (REGEXP_UNICODE(this)) result += 'u';
   if (REGEXP_STICKY(this)) result += 'y';
   return result;
 }
 
@@ skipping 457 matching lines @@
 // Exports
 
 utils.Export(function(to) {
   to.RegExpExec = DoRegExpExec;
   to.RegExpExecNoTests = RegExpExecNoTests;
   to.RegExpLastMatchInfo = RegExpLastMatchInfo;
   to.RegExpTest = RegExpTest;
 });
 
 })