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

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
diff --git a/runtime/vm/code_generator.cc b/runtime/vm/code_generator.cc
index 683834bd328178fc4de2f6c62711713ec3a4bb6c..69d5c603a9cc2a73778d5f5d264b2431d891b3fd 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -63,6 +63,7 @@ DEFINE_FLAG(charp, deoptimize_filter, NULL,
 
 DECLARE_FLAG(int, reload_every);
 DECLARE_FLAG(bool, reload_every_optimized);
+DECLARE_FLAG(bool, reload_every_back_off);
 
 #ifdef DEBUG
 DEFINE_FLAG(charp, gc_at_instance_allocation, NULL,
@@ -1297,6 +1298,9 @@ DEFINE_RUNTIME_ENTRY(StackOverflow, 0) {
     DeoptimizeFunctionsOnStack();
   }
   if (do_reload) {
+    if (FLAG_reload_every_back_off) {
+      FLAG_reload_every *= 2;
+    }
     NOT_IN_PRODUCT(isolate->ReloadSources();)
   }
   if (FLAG_support_debugger && do_stacktrace) {