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 75e0952581b3723aea67e4e9b108534d926fec86..9f7b24b6318f841a41adbf8a78670fc6dda04b2a 100644
--- a/src/runtime/runtime-i18n.cc
+++ b/src/runtime/runtime-i18n.cc
@@ -948,6 +948,8 @@ 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) {
@@ -1159,7 +1161,7 @@ RUNTIME_FUNCTION(Runtime_StringToUpperCaseI18N) {
   int32_t length = s->length();
   s = String::Flatten(s);
 
-  if (s->HasOnlyOneByteChars()) {
+  if (s->HasOnlyOneByteChars() && length > 0) {
     Handle<SeqOneByteString> result;
     ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
         isolate, result, isolate->factory()->NewRawOneByteString(length));