Make let/const outside of the extended mode early errors (under harmony flag).

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 2df734cecc19634bff62c6b00428efcc0e5230ea..92b0d3a3e846f7749911e00bdd83aedd8ced8b89 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"],