Revert "Proper sequencing of _asyncStackTraceHelper in Kernel"

runtime/lib/stacktrace.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "lib/stacktrace.h"
 #include "vm/bootstrap_natives.h"
 #include "vm/debugger.h"
 #include "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object_store.h"
@@ skipping 93 matching lines @@
   return CurrentStackTrace(thread, false, 0);
 }
 
 
 DEFINE_NATIVE_ENTRY(StackTrace_current, 0) {
   return CurrentStackTrace(thread, false);
 }
 
 
 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);
 }
 
 
 DEFINE_NATIVE_ENTRY(StackTrace_clearAsyncThreadStackTrace, 0) {
   thread->clear_async_stack_trace();
   return Object::null();
 }
@@ skipping 68 matching lines @@
                             Thread::Current(),
                             StackFrameIterator::kNoCrossThreadIteration);
   StackFrame* frame = frames.NextFrame();
   while (frame != NULL) {
     OS::PrintErr("%s\n", frame->ToCString());
     frame = frames.NextFrame();
   }
 }
 
 }  // namespace dart