Remove the ability to allow multiple gc prologue and gc epilogue callbacks

runtime/vm/isolate.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 "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "lib/mirrors.h"
@@ skipping 304 matching lines @@
       simulator_(NULL),
       long_jump_base_(NULL),
       timer_list_(),
       deopt_id_(0),
       mutex_(new Mutex()),
       stack_limit_(0),
       saved_stack_limit_(0),
       message_handler_(NULL),
       spawn_state_(NULL),
       is_runnable_(false),
-      gc_prologue_callbacks_(),
-      gc_epilogue_callbacks_(),
+      gc_prologue_callback_(NULL),
+      gc_epilogue_callback_(NULL),
       defer_finalization_count_(0),
       deopt_context_(NULL),
       stacktrace_(NULL),
       stack_frame_index_(-1),
       cha_used_(false),
       object_id_ring_(NULL),
       profiler_data_(NULL),
       thread_state_(NULL),
       next_(NULL),
       REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_INITIALIZERS)
@@ skipping 748 matching lines @@
   return func.raw();
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(isolate());
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart