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..c5fa131ad2cee1763a9ac42a05c4d6798a13a610 100644
--- a/runtime/lib/stacktrace.cc
+++ b/runtime/lib/stacktrace.cc
@@ -110,7 +110,7 @@ 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();
@@ -118,6 +118,7 @@ DEFINE_NATIVE_ENTRY(StackTrace_asyncStackTraceHelper, 1) {
       debugger->MaybeAsyncStepInto(async_op);
     }
   }
+#endif
   return CurrentStackTrace(thread, true);
 }