Guard @@isConcatSpreadable behind a flag

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
@@ skipping 255 matching lines @@
   # changes.
   inputs = [ "tools/jsmin.py" ]
 
   sources = [
     "src/macros.py",
     "src/messages.h",
     "src/proxy.js",
     "src/generator.js",
     "src/harmony-atomics.js",
     "src/harmony-array-includes.js",
+    "src/harmony-concat-spreadable.js",
     "src/harmony-tostring.js",
     "src/harmony-regexp.js",
     "src/harmony-reflect.js",
     "src/harmony-spread.js",
     "src/harmony-object.js",
     "src/harmony-sharedarraybuffer.js"
   ]
 
   outputs = [
     "$target_gen_dir/experimental-libraries.cc",
@@ skipping 1426 matching lines @@
         "src/d8-debug.cc",
         "src/d8-debug.h",
         "$target_gen_dir/d8-js.cc",
       ]
     }
     if (v8_enable_i18n_support) {
       deps += [ "//third_party/icu" ]
     }
   }
 }