Revert "Proper sequencing of _asyncStackTraceHelper in Kernel"

runtime/lib/stacktrace.cc

Index: runtime/lib/stacktrace.cc
diff --git a/runtime/lib/stacktrace.cc b/runtime/lib/stacktrace.cc
index a47bdb5be2567cb546d8ba4b86f189000aa0f720..48b87d4c50d5402a8ac8f4bc8d19b80cbab7eac6 100644
--- a/runtime/lib/stacktrace.cc
+++ b/runtime/lib/stacktrace.cc
@@ -111,13 +111,13 @@ DEFINE_NATIVE_ENTRY(StackTrace_current, 0) {
 
 
 DEFINE_NATIVE_ENTRY(StackTrace_asyncStackTraceHelper, 1) {
-  if (!FLAG_causal_async_stacks) return Object::null();
-
   GET_NATIVE_ARGUMENT(Closure, async_op, arguments->NativeArgAt(0));
-  if (FLAG_support_debugger) {
-    Debugger* debugger = isolate->debugger();
-    if (debugger != NULL) {
-      debugger->MaybeAsyncStepInto(async_op);
+  if (!async_op.IsNull()) {
+    if (FLAG_support_debugger) {
+      Debugger* debugger = isolate->debugger();
+      if (debugger != NULL) {
+        debugger->MaybeAsyncStepInto(async_op);
+      }
     }
   }
   return CurrentStackTrace(thread, true);