[Object.observe] Don't force normalization of elements for observed objects

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 454a1e8cfd9cd2ee42465ca192b65b06c72e22c3..578d22fa075e1373bc0ebb79500c8db0c1de3672 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -5627,12 +5627,6 @@ void JSObject::SetObserved(Handle<JSObject> object) {
   if (object->map()->is_observed())
     return;
 
-  if (!object->HasExternalArrayElements()) {
-    // Go to dictionary mode, so that we don't skip map checks.
-    NormalizeElements(object);
-    ASSERT(!object->HasFastElements());
-  }
-
   LookupResult result(isolate);
   object->map()->LookupTransition(*object,
                                   isolate->heap()->observed_symbol(),
@@ -5646,7 +5640,7 @@ void JSObject::SetObserved(Handle<JSObject> object) {
     new_map = Map::CopyForObserved(handle(object->map()));
   } else {
     new_map = Map::Copy(handle(object->map()));
-    new_map->set_is_observed(true);
+    new_map->set_is_observed();
   }
   object->set_map(*new_map);
 }
@@ -6951,7 +6945,7 @@ Handle<Map> Map::CopyForObserved(Handle<Map> map) {
 
   map->set_transitions(*transitions);
 
-  new_map->set_is_observed(true);
+  new_map->set_is_observed();
 
   if (map->owns_descriptors()) {
     new_map->InitializeDescriptors(map->instance_descriptors());
@@ -11136,7 +11130,6 @@ MaybeObject* JSObject::SetFastElementsCapacityAndLength(
   Heap* heap = GetHeap();
   // We should never end in here with a pixel or external array.
   ASSERT(!HasExternalArrayElements());
-  ASSERT(!map()->is_observed());
 
   // Allocate a new fast elements backing store.
   FixedArray* new_elements;
@@ -11221,7 +11214,6 @@ MaybeObject* JSObject::SetFastDoubleElementsCapacityAndLength(
   Heap* heap = GetHeap();
   // We should never end in here with a pixel or external array.
   ASSERT(!HasExternalArrayElements());
-  ASSERT(!map()->is_observed());
 
   FixedArrayBase* elems;
   { MaybeObject* maybe_obj =
@@ -11370,10 +11362,10 @@ MaybeObject* JSArray::SetElementsLength(Object* len) {
   if (!new_length_handle->ToArrayIndex(&new_length))
     return Failure::InternalError();
 
+  // TODO(adamk): The below used to be true but is no longer.
   // Observed arrays should always be in dictionary mode;
   // if they were in fast mode, the below is slower than necessary
   // as it iterates over the array backing store multiple times.
-  ASSERT(self->HasDictionaryElements());
   static const PropertyAttributes kNoAttrFilter = NONE;
   int num_elements = self->NumberOfLocalElements(kNoAttrFilter);
   if (num_elements > 0) {
@@ -12770,7 +12762,6 @@ MaybeObject* JSObject::UpdateAllocationSite(ElementsKind to_kind) {
 
 
 MaybeObject* JSObject::TransitionElementsKind(ElementsKind to_kind) {
-  ASSERT(!map()->is_observed());
   ElementsKind from_kind = map()->elements_kind();
 
   if (IsFastHoleyElementsKind(from_kind)) {