Add --hot-reload-test-mode flag to embedder and wire it up to test.py

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index c883ea52094c8e64f43a0cf4fb65cd2c87f5bac8..eee6d049a5dfdf039ba1fcd1549e764968d2b206 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -55,6 +55,7 @@ DECLARE_FLAG(bool, timing);
 DECLARE_FLAG(bool, trace_service);
 DECLARE_FLAG(bool, trace_reload);
 DECLARE_FLAG(bool, warn_on_pause_with_no_debugger);
+DECLARE_FLAG(bool, check_reloaded);
 
 NOT_IN_PRODUCT(
 static void CheckedModeHandler(bool value) {
@@ -1696,6 +1697,15 @@ void Isolate::Shutdown() {
     }
   }
 
+  if (FLAG_check_reloaded &&
+      (this != Dart::vm_isolate()) &&
+      !ServiceIsolate::IsServiceIsolateDescendant(this)) {
+    if (!HasAttemptedReload()) {
+      FATAL("Isolate did not reload before exiting and "
+            "--check-reloaded is enabled.\n");
+    }
+  }
+
   // Then, proceed with low-level teardown.
   LowLevelShutdown();