Split off a separate --harmony_sloppy_let flag

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 8d1bd881a63e7fed137d767fe508f454dfc04baf..027880cd0504454cd8f53e1947f89407fbb4671c 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -913,6 +913,7 @@ Parser::Parser(ParseInfo* info)
   set_allow_harmony_modules(!info->is_native() && FLAG_harmony_modules);
   set_allow_harmony_arrow_functions(FLAG_harmony_arrow_functions);
   set_allow_harmony_sloppy(FLAG_harmony_sloppy);
+  set_allow_harmony_sloppy_let(FLAG_harmony_sloppy_let);
   set_allow_harmony_unicode(FLAG_harmony_unicode);
   set_allow_harmony_computed_property_names(
       FLAG_harmony_computed_property_names);
@@ -4459,6 +4460,7 @@ PreParser::PreParseResult Parser::ParseLazyFunctionBodyWithPreParser(
     SET_ALLOW(harmony_modules);
     SET_ALLOW(harmony_arrow_functions);
     SET_ALLOW(harmony_sloppy);
+    SET_ALLOW(harmony_sloppy_let);
     SET_ALLOW(harmony_unicode);
     SET_ALLOW(harmony_computed_property_names);
     SET_ALLOW(harmony_rest_params);