Guard @@isConcatSpreadable behind a flag

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 181 matching lines @@
   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")             \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")     \
   V(harmony_atomics, "harmony atomics")                         \
   V(harmony_new_target, "harmony new.target")
 
 // Features that are complete (but still behind --harmony/es-staging flag).
-#define HARMONY_STAGED(V)                 \
-  V(harmony_tostring, "harmony toString") \
+#define HARMONY_STAGED(V)                                       \
+  V(harmony_tostring, "harmony toString")                       \
+  V(harmony_concat_spreadable, "harmony isConcatSpreadable")    \
   V(harmony_rest_parameters, "harmony rest parameters")
 
 // Features that are shipping (turned on by default, but internal flag remains).
 #define HARMONY_SHIPPING(V)                                             \
   V(harmony_arrow_functions, "harmony arrow functions")                 \
   V(harmony_computed_property_names, "harmony computed property names") \
   V(harmony_spreadcalls, "harmony spread-calls")                        \
   V(harmony_spread_arrays, "harmony spread in array literals")          \
   V(harmony_unicode, "harmony unicode escapes")                         \
   V(harmony_object, "harmony Object methods")
@@ skipping 847 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