Version 3.5.6.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 6603185a3207bf330bf4c1a34d04de782bdbc5cd..58c9a5302560373123893868e9edc25bd4fab5c4 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -247,6 +247,7 @@ function FormatMessage(message) {
       strict_cannot_assign:         ["Cannot assign to read only '", "%0", "' in strict mode"],
       strict_poison_pill:           ["'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them"],
       strict_caller:                ["Illegal access to a strict mode caller function."],
+      unprotected_let:              ["Illegal let declaration in unprotected statement context."],
     };
   }
   var message_type = %MessageGetType(message);