Statically check for assignments to const in harmony mode.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 2df734cecc19634bff62c6b00428efcc0e5230ea..ffedd8b7935f63a8591273050d1e267db9b4575a 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -206,6 +206,7 @@ function FormatMessage(message) {
       "illegal_break",                ["Illegal break statement"],
       "illegal_continue",             ["Illegal continue statement"],
       "illegal_return",               ["Illegal return statement"],
+      "illegal_let",                  ["Illegal let declaration outside extended mode"],
       "error_loading_debugger",       ["Error loading debugger"],
       "no_input_to_regexp",           ["No input to ", "%0"],
       "invalid_json",                 ["String '", "%0", "' is not valid JSON"],
@@ -245,6 +246,7 @@ function FormatMessage(message) {
       "unprotected_const",            ["Illegal const declaration in unprotected statement context."],
       "cant_prevent_ext_external_array_elements", ["Cannot prevent extension of an object with external array elements"],
       "redef_external_array_element", ["Cannot redefine a property of an object with external array elements"],
+      "harmony_const_assign",         ["Assignment to constant variable."],
     ];
     var messages = { __proto__ : null };
     for (var i = 0; i < messagesDictionary.length; i += 2) {