[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..b6b0ad46daabea38a8714935de31a5d2eeb3917e 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_OBJECT(this)) {

[Toon Verwaest, 2016/02/10 10:11:21]

This should be IS_RECEIVER or so. IS_OBJECT is just typeof this == "object",
which returns true for null.

Please add a test for this. As well as for other non-Object receivers with valid
.flags/.pattern functions on their intrinsicDefaultProto.

+      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';