Remove the debugger_ field from Isolate in a PRODUCT build

runtime/lib/async.cc

 // Copyright (c) 2017, 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 "vm/bootstrap_natives.h"
 #include "vm/debugger.h"
 #include "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object_store.h"
 #include "vm/runtime_entry.h"
 
 namespace dart {
 
 DEFINE_NATIVE_ENTRY(AsyncStarMoveNext_debuggerStepCheck, 1) {
+#if !defined(PRODUCT)
   GET_NON_NULL_NATIVE_ARGUMENT(Closure, async_op, arguments->NativeArgAt(0));
-  if (FLAG_support_debugger) {
-    Debugger* debugger = isolate->debugger();
-    if (debugger != NULL) {
-      debugger->MaybeAsyncStepInto(async_op);
-    }
+  Debugger* debugger = isolate->debugger();
+  if (debugger != NULL) {
+    debugger->MaybeAsyncStepInto(async_op);
   }
+#endif
   return Object::null();
 }
 
 }  // namespace dart