Object.observe: Make array length and other magic data properties work correctly.

src/property.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 292 matching lines @@
   bool IsFound() { return lookup_type_ != NOT_FOUND; }
   bool IsTransition() { return lookup_type_ == TRANSITION_TYPE; }
   bool IsHandler() { return lookup_type_ == HANDLER_TYPE; }
   bool IsInterceptor() { return lookup_type_ == INTERCEPTOR_TYPE; }
 
   // Is the result is a property excluding transitions and the null descriptor?
   bool IsProperty() {
     return IsFound() && !IsTransition();
   }
 
+  bool IsDataProperty() {
+    switch (type()) {
+      case FIELD:
+      case NORMAL:
+      case CONSTANT_FUNCTION:
+        return true;
+      case CALLBACKS: {
+        Object* callback = GetCallbackObject();
+        return callback->IsAccessorInfo() || callback->IsForeign();
+      }
+      case HANDLER:
+      case INTERCEPTOR:
+      case TRANSITION:
+      case NONEXISTENT:
+        return false;
+    }
+    UNREACHABLE();
+    return false;
+  }
+
   bool IsCacheable() { return cacheable_; }
   void DisallowCaching() { cacheable_ = false; }
 
   Object* GetLazyValue() {
     switch (type()) {
       case FIELD:
         return holder()->FastPropertyAt(GetFieldIndex().field_index());
       case NORMAL: {
         Object* value;
         value = holder()->property_dictionary()->ValueAt(GetDictionaryEntry());
         if (holder()->IsGlobalObject()) {
           value = JSGlobalPropertyCell::cast(value)->value();
         }
         return value;
       }
       case CONSTANT_FUNCTION:
         return GetConstantFunction();
-      default:
+      case CALLBACKS:
+      case HANDLER:
+      case INTERCEPTOR:
+      case TRANSITION:
+      case NONEXISTENT:
         return Isolate::Current()->heap()->the_hole_value();
     }
+    UNREACHABLE();
+    return NULL;
   }
 
   Map* GetTransitionTarget() {
     ASSERT(IsTransition());
     TransitionArray* transitions = holder()->map()->transitions();
     return transitions->GetTarget(number_);
   }
 
   PropertyDetails GetTransitionDetails(Map* map) {
     ASSERT(IsTransition());
@@ skipping 104 matching lines @@
   JSReceiver* holder_;
   int number_;
   bool cacheable_;
   PropertyDetails details_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_PROPERTY_H_