Add a new Sentinel type for sentinel responses. (They are not Null).

runtime/vm/object.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 13579 matching lines @@
   const intptr_t id = ring->GetIdForObject(raw());
   jsobj.AddPropertyF("id", "objects/%" Pd "", id);
 }
 
 
 void Instance::PrintJSONImpl(JSONStream* stream, bool ref) const {
   JSONObject jsobj(stream);
 
   // Handle certain special instance values.
   if (raw() == Object::sentinel().raw()) {
-    jsobj.AddProperty("type", ref ? "@Null" : "Null");
+    jsobj.AddProperty("type", "Sentinel");
     jsobj.AddProperty("id", "objects/not-initialized");
     jsobj.AddProperty("valueAsString", "<not initialized>");
     return;
   } else if (raw() == Object::transition_sentinel().raw()) {
-    jsobj.AddProperty("type", ref ? "@Null" : "Null");
+    jsobj.AddProperty("type", "Sentinel");
     jsobj.AddProperty("id", "objects/being-initialized");
     jsobj.AddProperty("valueAsString", "<being initialized>");
     return;
   }
 
   PrintSharedInstanceJSON(&jsobj, ref);
   ObjectIdRing* ring = Isolate::Current()->object_id_ring();
   const intptr_t id = ring->GetIdForObject(raw());
   if (IsClosure()) {
     const Function& closureFunc = Function::Handle(Closure::function(*this));
@@ skipping 3525 matching lines @@
   return buffer;
 }
 
 
 void String::PrintJSONImpl(JSONStream* stream, bool ref) const {
   JSONObject jsobj(stream);
   if (raw() == Symbols::OptimizedOut().raw()) {
     // TODO(turnidge): This is a hack.  The user could have this
     // special string in their program.  Fixing this involves updating
     // the debugging api a bit.
-    jsobj.AddProperty("type", ref ? "@Null" : "Null");
+    jsobj.AddProperty("type", "Sentinel");
     jsobj.AddProperty("id", "objects/optimized-out");
     jsobj.AddProperty("valueAsString", "<optimized out>");
     return;
   }
   PrintSharedInstanceJSON(&jsobj, ref);
   ObjectIdRing* ring = Isolate::Current()->object_id_ring();
   const intptr_t id = ring->GetIdForObject(raw());
   jsobj.AddPropertyF("id", "objects/%" Pd "", id);
   jsobj.AddProperty("valueAsString", ToUserCString(1024));
 }
@@ skipping 2418 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart