Revert "Handlify concat string and substring."

src/handles.cc

Index: src/handles.cc
diff --git a/src/handles.cc b/src/handles.cc
index 5ef0c46438de31ea436c0373b98c638d13fd5513..f3928ebf779f7d98c404a032aa43c4bfdda6591e 100644
--- a/src/handles.cc
+++ b/src/handles.cc
@@ -229,12 +229,11 @@ Handle<Object> GetProperty(Isolate* isolate,
 }
 
 
-Handle<String> LookupSingleCharacterStringFromCode(Isolate* isolate,
+Handle<Object> LookupSingleCharacterStringFromCode(Isolate* isolate,
                                                    uint32_t index) {
   CALL_HEAP_FUNCTION(
       isolate,
-      isolate->heap()->LookupSingleCharacterStringFromCode(index),
-      String);
+      isolate->heap()->LookupSingleCharacterStringFromCode(index), Object);
 }