Split off a separate --harmony_sloppy_let flag

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index db05e3e8dfe0e26ab94235db1a0fdd7a40e4e6a8..71b61e6b52c573e4f64ea64926dee37a9d4c3b0b 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1864,6 +1864,7 @@ EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_regexps)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_arrow_functions)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_tostring)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_sloppy)
+EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_sloppy_let)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_unicode)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_unicode_regexps)
 EMPTY_NATIVE_FUNCTIONS_FOR_FEATURE(harmony_computed_property_names)
@@ -1900,6 +1901,7 @@ EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_array_includes)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_arrow_functions)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_proxies)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_sloppy)
+EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_sloppy_let)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_unicode)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_computed_property_names)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_rest_parameters)
@@ -2578,6 +2580,7 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_tostring_natives[] = {"native harmony-tostring.js",
                                                    nullptr};
   static const char* harmony_sloppy_natives[] = {nullptr};
+  static const char* harmony_sloppy_let_natives[] = {nullptr};
   static const char* harmony_unicode_natives[] = {nullptr};
   static const char* harmony_unicode_regexps_natives[] = {nullptr};
   static const char* harmony_computed_property_names_natives[] = {nullptr};