Remove the debugger_ field from Isolate in a PRODUCT build

runtime/lib/stacktrace.cc

Index: runtime/lib/stacktrace.cc
diff --git a/runtime/lib/stacktrace.cc b/runtime/lib/stacktrace.cc
index c5c41f2573d9f4c6e9cd0624e959e60ada688cbc..a15b9bd99b3dfc77882c38cc6e6a7e012e1e9b78 100644
--- a/runtime/lib/stacktrace.cc
+++ b/runtime/lib/stacktrace.cc
@@ -110,14 +110,13 @@ DEFINE_NATIVE_ENTRY(StackTrace_asyncStackTraceHelper, 1) {
   if (!FLAG_causal_async_stacks) {
     return Object::null();
   }
-
+#if !defined(PRODUCT)
   GET_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 CurrentStackTrace(thread, true);
 }