Internalize strings in-place

src/runtime/runtime-strings.cc

Index: src/runtime/runtime-strings.cc
diff --git a/src/runtime/runtime-strings.cc b/src/runtime/runtime-strings.cc
index 31d9f1fc6ede906c8cae1d4be3036db6a7336107..a126e8cd6379a92f766636036da5e9dcfc2903d6 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -263,6 +263,9 @@ RUNTIME_FUNCTION(Runtime_StringBuilderConcat) {
   if (length == -1) {
     return isolate->Throw(isolate->heap()->illegal_argument_string());
   }
+  if (length == 0) {
+    return isolate->heap()->empty_string();
+  }
 
   if (one_byte) {
     Handle<SeqOneByteString> answer;