Reland "Track field types.".

src/ic.cc

Index: src/ic.cc
diff --git a/src/ic.cc b/src/ic.cc
index fef90e43f68a331bc63c44f44cb702e470f2b6d2..e10b0fdc6139f5a5117c290228013e5f62f37b39 100644
--- a/src/ic.cc
+++ b/src/ic.cc
@@ -1231,9 +1231,12 @@ static bool LookupForWrite(Handle<JSObject> receiver,
   ASSERT(!receiver->map()->is_deprecated());
   if (!lookup->CanHoldValue(value)) {
     Handle<Map> target(lookup->GetTransitionTarget());
+    Representation field_representation = value->OptimalRepresentation();
+    Handle<HeapType> field_type = value->OptimalType(
+        lookup->isolate(), field_representation);
     Map::GeneralizeRepresentation(
         target, target->LastAdded(),
-        value->OptimalRepresentation(), FORCE_FIELD);
+        field_representation, field_type, FORCE_FIELD);
     // Lookup the transition again since the transition tree may have changed
     // entirely by the migration above.
     receiver->map()->LookupTransition(*holder, *name, lookup);