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

runtime/vm/service.cc

 // Copyright (c) 2013, 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/service.h"
 
 #include "include/dart_api.h"
 #include "platform/globals.h"
 
 #include "vm/compiler.h"
@@ skipping 1102 matching lines @@
       PrintError(js, "attempt to evaluate against internal VM object\n");
       return true;
     }
     const char* expr = js->LookupOption("expr");
     if (expr == NULL) {
       PrintError(js, "eval expects an 'expr' option\n",
                  js->num_arguments());
       return true;
     }
     const String& expr_str = String::Handle(isolate, String::New(expr));
-    ASSERT(obj->IsInstance());
-    const Instance& instance = Instance::Cast(*obj);
+    ASSERT(obj->IsInstance() || obj->IsNull());
+    Instance& instance = Instance::Handle();
+    instance ^= obj->raw();
     const Object& result =
         Object::Handle(instance.Evaluate(expr_str,
                                          Array::empty_array(),
                                          Array::empty_array()));
     result.PrintJSON(js, true);
     return true;
   } else if (strcmp(action, "retained") == 0) {
     if (kind == ObjectIdRing::kCollected) {
       PrintErrorWithKind(
           js, "RetainedCollected",
@@ skipping 530 matching lines @@
     } else {
       PrintError(js, "Invalid sub collection %s", second);
       return true;
     }
   }
   UNREACHABLE();
   return true;
 }
 
 
-static void PrintPseudoNull(JSONStream* js,
-                            const char* id,
-                            const char* preview) {
+static void PrintSentinel(JSONStream* js,
+                          const char* id,
+                          const char* preview) {
   JSONObject jsobj(js);
-  jsobj.AddProperty("type", "Null");
+  jsobj.AddProperty("type", "Sentinel");
   jsobj.AddProperty("id", id);
   jsobj.AddProperty("valueAsString", preview);
 }
 
 
 static RawObject* LookupObjectId(Isolate* isolate,
                                  const char* arg,
                                  ObjectIdRing::LookupResult* kind) {
   *kind = ObjectIdRing::kValid;
   if (strncmp(arg, "int-", 4) == 0) {
@@ skipping 167 matching lines @@
     } else {
       Symbols::OptimizedOut().PrintJSON(js, false);
     }
     return true;
 
   } else if (strcmp(arg, "collected") == 0) {
     if (js->num_arguments() > 2) {
       PrintError(js, "expected at most 2 arguments but found %" Pd "\n",
                  js->num_arguments());
     } else {
-      PrintPseudoNull(js, "objects/collected", "<collected>");
+      PrintSentinel(js, "objects/collected", "<collected>");
     }
     return true;
 
   } else if (strcmp(arg, "expired") == 0) {
     if (js->num_arguments() > 2) {
       PrintError(js, "expected at most 2 arguments but found %" Pd "\n",
                  js->num_arguments());
     } else {
-      PrintPseudoNull(js, "objects/expired", "<expired>");
+      PrintSentinel(js, "objects/expired", "<expired>");
     }
     return true;
   }
 
   // Lookup the object.
   Object& obj = Object::Handle(isolate);
   ObjectIdRing::LookupResult kind = ObjectIdRing::kInvalid;
   obj = LookupObjectId(isolate, arg, &kind);
   if (kind == ObjectIdRing::kInvalid) {
     PrintError(js, "unrecognized object id '%s'", arg);
     return true;
   }
   if (js->num_arguments() == 2) {
     // Print.
     if (kind == ObjectIdRing::kCollected) {
       // The object has been collected by the gc.
-      PrintPseudoNull(js, "objects/collected", "<collected>");
+      PrintSentinel(js, "objects/collected", "<collected>");
       return true;
     } else if (kind == ObjectIdRing::kExpired) {
       // The object id has expired.
-      PrintPseudoNull(js, "objects/expired", "<expired>");
+      PrintSentinel(js, "objects/expired", "<expired>");
       return true;
     }
     obj.PrintJSON(js, false);
     return true;
   }
   return HandleInstanceCommands(isolate, &obj, kind, js, 2);
 }
 
 
 static bool HandleScriptsEnumerate(Isolate* isolate, JSONStream* js) {
@@ skipping 801 matching lines @@
   while (current != NULL) {
     if (strcmp(name, current->name()) == 0) {
       return current;
     }
     current = current->next();
   }
   return NULL;
 }
 
 }  // namespace dart