Do not generalize field representations when making elements kind or observed transition.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 49f48a2c0853b6122ded30d9e6525818db82962b..c8d063c677c3a475482c96de1e7b1f89e57fdeb5 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6144,6 +6144,11 @@ class Map: public HeapObject {
 
   inline void AppendDescriptor(Descriptor* desc);
 
+  // Returns a copy of the map, prepared for inserting into the transition
+  // tree (if the |map| owns descriptors then the new one will share
+  // descriptors with |map|).
+  static Handle<Map> CopyForTransition(Handle<Map> map, const char* reason);
+
   // Returns a copy of the map, with all transitions dropped from the
   // instance descriptors.
   static Handle<Map> Copy(Handle<Map> map, const char* reason);