Make `let` usable as an identifier in ES6 sloppy mode.

src/messages.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // -------------------------------------------------------------------
 
 var kMessages = {
   // Error
   cyclic_proto:                  ["Cyclic __proto__ value"],
   code_gen_from_strings:         ["%0"],
@@ skipping 109 matching lines @@
   invalid_lhs_in_prefix_op:      ["Invalid left-hand side expression in prefix operation"],
   // SyntaxError
   paren_in_arg_string:           ["Function arg string contains parenthesis"],
   not_isvar:                     ["builtin %IS_VAR: not a variable"],
   single_function_literal:       ["Single function literal required"],
   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"],
   called_on_non_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_cached_data_function:  ["Invalid cached data for function ", "%0"],
@@ skipping 1141 matching lines @@
 function SetUpStackOverflowBoilerplate() {
   var boilerplate = MakeRangeError('stack_overflow', []);
 
   %DefineAccessorPropertyUnchecked(
       boilerplate, 'stack', StackTraceGetter, StackTraceSetter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();