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

src/js/regexp.js

Index: src/js/regexp.js
diff --git a/src/js/regexp.js b/src/js/regexp.js
index a456952ef37ba6ee5797f546256f904e551ec900..f4c6f8a6cee8c09c00e2551be127ae42677e0567 100644
--- a/src/js/regexp.js
+++ b/src/js/regexp.js
@@ -278,8 +278,11 @@ function RegExpToString() {
       %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';