Turn sliced strings into flat cons-strings upon internalization

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 37827033a8c66f317f40854928964963413d8934..c20069f2d931feee4615ddcb81083433ae10f2c1 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -17232,6 +17232,16 @@ Handle<String> StringTable::LookupString(Isolate* isolate,
     Handle<ConsString> cons = Handle<ConsString>::cast(string);
     cons->set_first(*result);
     cons->set_second(isolate->heap()->empty_string());
+  } else if (string->IsSlicedString()) {
+    STATIC_ASSERT(ConsString::kSize == SlicedString::kSize);
+    DisallowHeapAllocation no_gc;
+    bool one_byte = result->IsOneByteRepresentation();
+    Handle<Map> map = one_byte ? isolate->factory()->cons_one_byte_string_map()
+                               : isolate->factory()->cons_string_map();
+    string->set_map(*map);
+    Handle<ConsString> cons = Handle<ConsString>::cast(string);
+    cons->set_first(*result);
+    cons->set_second(isolate->heap()->empty_string());
   }
   return result;
 }