Turn message property of the error object into a data property.

test/mjsunit/harmony/proxies.js

Index: test/mjsunit/harmony/proxies.js
diff --git a/test/mjsunit/harmony/proxies.js b/test/mjsunit/harmony/proxies.js
index 7170ffd9c73e25980d3ebcb99638f1894fbf41f9..04fc76949e4200a01425368e0e30f20abd5508a1 100644
--- a/test/mjsunit/harmony/proxies.js
+++ b/test/mjsunit/harmony/proxies.js
@@ -649,6 +649,11 @@ function TestSetForDerived2(create, trap) {
 
 TestSetForDerived(
   function(k) {
+    // TODO(yangguo): issue 2398 - throwing an error causes formatting of
+    // the message string, which can be observable through this handler.
+    // We ignore keys that occur when formatting the message string.
+    if (k == "toString" || k == "valueOf") return;
+
     key = k;
     switch (k) {
       case "p_writable": return {writable: true, configurable: true}