Improve internal stringifcation for custom Error objects.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index b586d24882b02a5e46528efef9dded04ac510540..1561e4abab0011d2dc704e3129ebc7b9255b8dcc 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -228,7 +228,11 @@ function NoSideEffectToString(obj) {
       }
     }
   }
-  if (IsNativeErrorObject(obj)) return %_CallFunction(obj, ErrorToString);
+  var objToString = GetPropertyWithoutInvokingMonkeyGetters(obj, "toString");

[Michael Starzinger, 2013/08/02 12:16:02]

Yep, better, final nit: Can we move this into the "IsNativeErrorObject" helper
to not duplicate it at all use-sites of that helper?

+  if (IsNativeErrorObject(obj) ||
+      (obj instanceof $Error && objToString === ErrorToString)) {
+    return %_CallFunction(obj, ErrorToString);
+  }
   return %_CallFunction(obj, ObjectToString);
 }
 
@@ -257,7 +261,9 @@ function IsNativeErrorObject(obj) {
 // the error to string method. This is to avoid leaking error
 // objects between script tags in a browser setting.
 function ToStringCheckErrorObject(obj) {
-  if (IsNativeErrorObject(obj)) {
+  var objToString = GetPropertyWithoutInvokingMonkeyGetters(obj, "toString");
+  if (IsNativeErrorObject(obj) ||
+      (obj instanceof $Error && objToString === ErrorToString)) {
     return %_CallFunction(obj, ErrorToString);
   } else {
     return ToString(obj);