Remove the debugger_ field from Isolate in a PRODUCT build

runtime/lib/stacktrace.cc

 // Copyright (c) 2013, 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 "lib/stacktrace.h"
 #include "vm/bootstrap_natives.h"
 #include "vm/debugger.h"
 #include "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object_store.h"
@@ skipping 92 matching lines @@
 }
 
 DEFINE_NATIVE_ENTRY(StackTrace_current, 0) {
   return CurrentStackTrace(thread, false);
 }
 
 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);
     }
   }
+#endif
   return CurrentStackTrace(thread, true);
 }
 
 DEFINE_NATIVE_ENTRY(StackTrace_clearAsyncThreadStackTrace, 0) {
   thread->clear_async_stack_trace();
   return Object::null();
 }
 
 DEFINE_NATIVE_ENTRY(StackTrace_setAsyncThreadStackTrace, 1) {
   if (!FLAG_causal_async_stacks) {
@@ skipping 42 matching lines @@
   AppendFrames(code_list, pc_offset_list, skip_frames);
   const Array& code_array = Array::Handle(Array::MakeFixedLength(code_list));
   const Array& pc_offset_array =
       Array::Handle(Array::MakeFixedLength(pc_offset_list));
   const StackTrace& stacktrace =
       StackTrace::Handle(StackTrace::New(code_array, pc_offset_array));
   return stacktrace;
 }
 
 }  // namespace dart