Remove the debugger_ field from Isolate in a PRODUCT build

runtime/lib/async.cc

Index: runtime/lib/async.cc
diff --git a/runtime/lib/async.cc b/runtime/lib/async.cc
index 99219a9150ede6e00c5ee6122d4df197b661f6fd..a71c1a1081ce68e23aa75245fbb7583f213414af 100644
--- a/runtime/lib/async.cc
+++ b/runtime/lib/async.cc
@@ -12,6 +12,7 @@
 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();
@@ -19,6 +20,7 @@ DEFINE_NATIVE_ENTRY(AsyncStarMoveNext_debuggerStepCheck, 1) {
       debugger->MaybeAsyncStepInto(async_op);
     }
   }
+#endif
   return Object::null();
 }