Reland "Handlify concat string and substring."

src/handles.cc

Index: src/handles.cc
diff --git a/src/handles.cc b/src/handles.cc
index 2d414022e09203e8a1034ee24fd55ca49f699063..691ce9f90a42e5eb814a49ccf3893daee93052ba 100644
--- a/src/handles.cc
+++ b/src/handles.cc
@@ -211,11 +211,12 @@ Handle<Object> GetProperty(Isolate* isolate,
 }
 
 
-Handle<Object> LookupSingleCharacterStringFromCode(Isolate* isolate,
+Handle<String> LookupSingleCharacterStringFromCode(Isolate* isolate,
                                                    uint32_t index) {
   CALL_HEAP_FUNCTION(
       isolate,
-      isolate->heap()->LookupSingleCharacterStringFromCode(index), Object);
+      isolate->heap()->LookupSingleCharacterStringFromCode(index),
+      String);
 }