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

runtime/vm/service_test.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 "platform/globals.h"
 
 #include "include/dart_debugger_api.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
@@ skipping 516 matching lines @@
       "{\"type\":\"Null\",\"id\":\"objects\\/null\","
       "\"valueAsString\":\"null\",\"class\":",
       handler.msg());
 
   // not initialized
   service_msg = Eval(lib, "[0, port, ['objects', 'not-initialized'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/not-initialized\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/not-initialized\","
       "\"valueAsString\":\"<not initialized>\"}",
       handler.msg());
 
   // being initialized
   service_msg = Eval(lib,
                      "[0, port, ['objects', 'being-initialized'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/being-initialized\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/being-initialized\","
       "\"valueAsString\":\"<being initialized>\"}",
       handler.msg());
 
   // optimized out
   service_msg = Eval(lib, "[0, port, ['objects', 'optimized-out'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/optimized-out\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/optimized-out\","
       "\"valueAsString\":\"<optimized out>\"}",
       handler.msg());
 
   // collected
   service_msg = Eval(lib, "[0, port, ['objects', 'collected'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/collected\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/collected\","
       "\"valueAsString\":\"<collected>\"}",
       handler.msg());
 
   // expired
   service_msg = Eval(lib, "[0, port, ['objects', 'expired'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/expired\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/expired\","
       "\"valueAsString\":\"<expired>\"}",
       handler.msg());
 
   // bool
   service_msg = Eval(lib, "[0, port, ['objects', 'bool-true'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
       "{\"type\":\"Bool\",\"id\":\"objects\\/bool-true\","
@@ skipping 33 matching lines @@
           "\"class\":{\"type\":\"@Class\",\"name\":\"_OneByteString\",},"
           "\"valueAsString\":\"\\\"value\\\"\"}}]}",
       handler.msg());
 
   // object id ring / invalid => expired
   service_msg = Eval(lib, "[0, port, ['objects', '99999999'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/expired\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/expired\","
       "\"valueAsString\":\"<expired>\"}",
       handler.msg());
 
   // expired/eval => error
   service_msg = Eval(lib, "[0, port, ['objects', 'expired', 'eval'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
       "{\"type\":\"Error\",\"id\":\"\","
@@ skipping 1382 matching lines @@
   service_msg = Eval(lib, "[0, port, ['profile'], ['tags'], ['hidden']]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   // Expect error.
   EXPECT_SUBSTRING("\"type\":\"Error\"", handler.msg());
 }
 
 #endif  // !defined(TARGET_ARCH_ARM64)
 
 }  // namespace dart