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

src/messages.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 188 matching lines @@
       "invalid_array_length",         ["Invalid array length"],
       "stack_overflow",               ["Maximum call stack size exceeded"],
       "invalid_time_value",           ["Invalid time value"],
       // SyntaxError
       "unable_to_parse",              ["Parse error"],
       "invalid_regexp_flags",         ["Invalid flags supplied to RegExp constructor '", "%0", "'"],
       "invalid_regexp",               ["Invalid RegExp pattern /", "%0", "/"],
       "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"],
       "circular_structure",           ["Converting circular structure to JSON"],
       "obj_ctor_property_non_object", ["Object.", "%0", " called on non-object"],
       "called_on_null_or_undefined",  ["%0", " called on null or undefined"],
       "array_indexof_not_defined",    ["Array.getIndexOf: Argument undefined"],
       "object_not_extensible",        ["Can't add property ", "%0", ", object is not extensible"],
       "illegal_access",               ["Illegal access"],
       "invalid_preparser_data",       ["Invalid preparser data for function ", "%0"],
@@ skipping 971 matching lines @@
     throw e;
   }
 }
 
 
 InstallFunctions($Error.prototype, DONT_ENUM, ['toString', ErrorToString]);
 
 // Boilerplate for exceptions for stack overflows. Used from
 // Isolate::StackOverflow().
 const kStackOverflowBoilerplate = MakeRangeError('stack_overflow', []);