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

src/object-observe.js

 // 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 66 matching lines @@
   if (!observerInfoMap.has(callback)) {
     observerInfoMap.set(callback, {
       pendingChangeRecords: null,
       priority: observationState.observerPriority++,
     });
   }
 
   var objectInfo = objectInfoMap.get(object);
   if (IS_UNDEFINED(objectInfo)) {
     objectInfo = CreateObjectInfo(object);
-    %SetIsObserved(object, true);
   }
+  %SetIsObserved(object, true);
 
   var changeObservers = objectInfo.changeObservers;
   if (changeObservers.indexOf(callback) < 0)
     changeObservers.push(callback);
 
   return object;
 }
 
 function ObjectUnobserve(object, callback) {
   if (!IS_SPEC_OBJECT(object))
     throw MakeTypeError("observe_non_object", ["unobserve"]);
   if (!IS_SPEC_FUNCTION(callback))
     throw MakeTypeError("observe_non_function", ["unobserve"]);
 
   var objectInfo = objectInfoMap.get(object);
   if (IS_UNDEFINED(objectInfo))
     return object;
 
   var changeObservers = objectInfo.changeObservers;
   var index = changeObservers.indexOf(callback);
-  if (index >= 0)
+  if (index >= 0) {
     changeObservers.splice(index, 1);
+    if (changeObservers.length === 0)
+      %SetIsObserved(object, false);
+  }
 
   return object;
 }
 
 function EnqueueChangeRecord(changeRecord, observers) {
   for (var i = 0; i < observers.length; i++) {
     var observer = observers[i];
     var observerInfo = observerInfoMap.get(observer);
     observationState.pendingObservers[observerInfo.priority] = observer;
     %SetObserverDeliveryPending();
@@ skipping 112 matching lines @@
     "getNotifier", ObjectGetNotifier,
     "observe", ObjectObserve,
     "unobserve", ObjectUnobserve
   ));
   InstallFunctions(notifierPrototype, DONT_ENUM, $Array(
     "notify", ObjectNotifierNotify
   ));
 }
 
 SetupObjectObserve();