Guard @@isConcatSpreadable behind a flag

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 983da4787fe5a3064b3369e7e8f550544c8beeea..43fc0eb8355a7ad0d1be7ccb0fe4bd233ac81dd9 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1819,6 +1819,7 @@ EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_spread_arrays)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_sharedarraybuffer)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_atomics)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_new_target)
+EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_concat_spreadable)
 
 
 void Genesis::InstallNativeFunctions_harmony_proxies() {
@@ -1850,6 +1851,7 @@ EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_object)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_spread_arrays)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_atomics)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_new_target)
+EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_concat_spreadable)
 
 void Genesis::InitializeGlobal_harmony_regexps() {
   Handle<JSObject> builtins(native_context()->builtins());
@@ -2500,6 +2502,8 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_atomics_natives[] = {"native harmony-atomics.js",
                                                   nullptr};
   static const char* harmony_new_target_natives[] = {nullptr};
+  static const char* harmony_concat_spreadable_natives[] = {
+      "native harmony-concat-spreadable.js", nullptr};
 
   for (int i = ExperimentalNatives::GetDebuggerCount();
        i < ExperimentalNatives::GetBuiltinsCount(); i++) {