Remove the debugger_ field from Isolate in a PRODUCT build

runtime/vm/dart_entry.cc

Index: runtime/vm/dart_entry.cc
diff --git a/runtime/vm/dart_entry.cc b/runtime/vm/dart_entry.cc
index 880046533839020af52f698d5deacca0b02927ed..8d9a14d93ad5a6ecdb866d42ce8d1f6a0c7c5c4c 100644
--- a/runtime/vm/dart_entry.cc
+++ b/runtime/vm/dart_entry.cc
@@ -561,12 +561,14 @@ RawObject* DartLibraryCalls::HandleMessage(const Object& handler,
   const Array& args = Array::Handle(zone, Array::New(kNumArguments));
   args.SetAt(0, handler);
   args.SetAt(1, message);
+#if !defined(PRODUCT)
   if (FLAG_support_debugger && isolate->debugger()->IsStepping()) {
     // If the isolate is being debugged and the debugger was stepping
     // through code, enable single stepping so debugger will stop
     // at the first location the user is interested in.
     isolate->debugger()->SetResumeAction(Debugger::kStepInto);
   }
+#endif
   const Object& result =
       Object::Handle(zone, DartEntry::InvokeFunction(function, args));
   ASSERT(result.IsNull() || result.IsError());