Include 'fixedId' key when printing service ids

runtime/vm/json_test.cc

Index: runtime/vm/json_test.cc
diff --git a/runtime/vm/json_test.cc b/runtime/vm/json_test.cc
index b7c25dad4ad05f1a3e4ca3f8cab946a736f37554..d4bdf036aafe19f575478d728e7f4f97c8bf70f7 100644
--- a/runtime/vm/json_test.cc
+++ b/runtime/vm/json_test.cc
@@ -299,9 +299,11 @@ TEST_CASE(JSON_JSONStream_DartObject) {
     JSONObject jsobj(&jsarr);
     jsobj.AddProperty("object_key", Object::Handle(Object::null()));
   }
-  EXPECT_STREQ("[{\"type\":\"@null\",\"id\":\"objects\\/null\","
+  EXPECT_STREQ("[{\"type\":\"@null\",\"fixedId\":true,"
+               "\"id\":\"objects\\/null\","
                "\"valueAsString\":\"null\"},"
-               "{\"object_key\":{\"type\":\"@null\",\"id\":\"objects\\/null\","
+               "{\"object_key\":{\"type\":\"@null\",\"fixedId\":true,"
+               "\"id\":\"objects\\/null\","
                "\"valueAsString\":\"null\"}}]",
                js.ToCString());
 }