Remove one more case behind --es5_readonly flag.

test/mjsunit/with-readonly.js

Index: test/mjsunit/with-readonly.js
diff --git a/test/mjsunit/with-readonly.js b/test/mjsunit/with-readonly.js
index 43583348e95e9a59c617bad912b1413774ff33b4..29982b34746dd0136808154c5d5ad58960cb5212 100644
--- a/test/mjsunit/with-readonly.js
+++ b/test/mjsunit/with-readonly.js
@@ -27,6 +27,8 @@
 
 // Test that readonly variables are treated correctly.
 
+// Flags: --es5_readonly
+
 // Create an object with a read-only length property in the prototype
 // chain by putting the string split function in the prototype chain.
 var o = {};