Fix LoadFieldByIndex to take mutable heap-numbers into account.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 0013e720d48ea0f0389dc528feed442af8128bc3..168e1d623fd848d37fd1714652c0847766850f98 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -5640,6 +5640,15 @@ Handle<JSObject> JSObject::Copy(Handle<JSObject> object) {
 }
 
 
+Handle<Object> JSObject::FastPropertyAt(Handle<JSObject> object,
+                                        Representation representation,
+                                        int index) {
+  Isolate* isolate = object->GetIsolate();
+  CALL_HEAP_FUNCTION(isolate,
+                     object->FastPropertyAt(representation, index), Object);
+}
+
+
 template<class ContextObject>
 class JSObjectWalkVisitor {
  public: