Fix LoadFieldByIndex to take mutable heap-numbers into account.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 0e352a2e8bcfc744b2397d3da11f8219a776f607..b633e35c1857bca36b55fc7926a6fca0ba4989e1 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2613,6 +2613,9 @@ class JSObject: public JSReceiver {
   MUST_USE_RESULT inline MaybeObject* FastPropertyAt(
       Representation representation,
       int index);
+  static Handle<Object> FastPropertyAt(Handle<JSObject> object,
+                                       Representation representation,
+                                       int index);
   inline Object* RawFastPropertyAt(int index);
   inline void FastPropertyAtPut(int index, Object* value);