Add a flag for legacy const semantics

src/flag-definitions.h

 // 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.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 164 matching lines @@
 DEFINE_BOOL(strong_mode, false, "experimental strong language mode")
 DEFINE_IMPLICATION(use_strong, strong_mode)
 DEFINE_BOOL(strong_this, true, "don't allow 'this' to escape from constructors")
 
 DEFINE_BOOL(es_staging, false, "enable all completed harmony features")
 DEFINE_BOOL(harmony, false, "enable all completed harmony features")
 DEFINE_BOOL(harmony_shipping, true, "enable all shipped harmony fetaures")
 DEFINE_IMPLICATION(harmony, es_staging)
 DEFINE_IMPLICATION(es_staging, harmony)
 
+DEFINE_BOOL(legacy_const, true, "legacy semantics for const in sloppy mode")
+
 // Features that are still work in progress (behind individual flags).
 #define HARMONY_INPROGRESS(V)                                   \
   V(harmony_modules, "harmony modules")                         \
   V(harmony_array_includes, "harmony Array.prototype.includes") \
   V(harmony_regexps, "harmony regular expression extensions")   \
   V(harmony_proxies, "harmony proxies")                         \
   V(harmony_sloppy, "harmony features in sloppy mode")          \
   V(harmony_unicode_regexps, "harmony unicode regexps")         \
   V(harmony_reflect, "harmony Reflect API")                     \
   V(harmony_destructuring, "harmony destructuring")             \
@@ skipping 864 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA