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

runtime/vm/service_test.cc

Index: runtime/vm/service_test.cc
diff --git a/runtime/vm/service_test.cc b/runtime/vm/service_test.cc
index 5a0f5b260878e2cabb56fee94a6a17d60e79e759..bc7483783877fd2208a6262e6873fdf17d7707e1 100644
--- a/runtime/vm/service_test.cc
+++ b/runtime/vm/service_test.cc
@@ -534,7 +534,7 @@ TEST_CASE(Service_Objects) {
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/not-initialized\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/not-initialized\","
       "\"valueAsString\":\"<not initialized>\"}",
       handler.msg());
 
@@ -545,7 +545,7 @@ TEST_CASE(Service_Objects) {
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/being-initialized\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/being-initialized\","
       "\"valueAsString\":\"<being initialized>\"}",
       handler.msg());
 
@@ -555,7 +555,7 @@ TEST_CASE(Service_Objects) {
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/optimized-out\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/optimized-out\","
       "\"valueAsString\":\"<optimized out>\"}",
       handler.msg());
 
@@ -565,7 +565,7 @@ TEST_CASE(Service_Objects) {
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/collected\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/collected\","
       "\"valueAsString\":\"<collected>\"}",
       handler.msg());
 
@@ -575,7 +575,7 @@ TEST_CASE(Service_Objects) {
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/expired\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/expired\","
       "\"valueAsString\":\"<expired>\"}",
       handler.msg());
 
@@ -629,7 +629,7 @@ TEST_CASE(Service_Objects) {
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
-      "{\"type\":\"Null\",\"id\":\"objects\\/expired\","
+      "{\"type\":\"Sentinel\",\"id\":\"objects\\/expired\","
       "\"valueAsString\":\"<expired>\"}",
       handler.msg());