Internalize strings in-place

src/runtime/runtime-i18n.cc

Index: src/runtime/runtime-i18n.cc
diff --git a/src/runtime/runtime-i18n.cc b/src/runtime/runtime-i18n.cc
index 6630fadc10b2e3e829b0461101533da836c1eb93..0b45381914641a824e36e99eaa0d315bf96252aa 100644
--- a/src/runtime/runtime-i18n.cc
+++ b/src/runtime/runtime-i18n.cc
@@ -831,6 +831,8 @@ MUST_USE_RESULT Object* LocaleConvertCase(Handle<String> s, Isolate* isolate,
   Handle<SeqTwoByteString> result;
   std::unique_ptr<uc16[]> sap;
 
+  if (dest_length == 0) return isolate->heap()->empty_string();
+
   // This is not a real loop. It'll be executed only once (no overflow) or
   // twice (overflow).
   for (int i = 0; i < 2; ++i) {
@@ -1041,7 +1043,7 @@ MUST_USE_RESULT Object* ConvertToLower(Handle<String> s, Isolate* isolate) {
 
 MUST_USE_RESULT Object* ConvertToUpper(Handle<String> s, Isolate* isolate) {
   int32_t length = s->length();
-  if (s->HasOnlyOneByteChars()) {
+  if (s->HasOnlyOneByteChars() && length > 0) {
     Handle<SeqOneByteString> result =
         isolate->factory()->NewRawOneByteString(length).ToHandleChecked();