Remove GC prologue and 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 "include/dart_native_api.h"
 #include "platform/assert.h"
 #include "platform/text_buffer.h"
@@ skipping 781 matching lines @@
       api_state_(NULL),
       random_(),
       simulator_(NULL),
       mutex_(new Mutex()),
       symbols_mutex_(new Mutex()),
       type_canonicalization_mutex_(new Mutex()),
       constant_canonicalization_mutex_(new Mutex()),
       megamorphic_lookup_mutex_(new Mutex()),
       message_handler_(NULL),
       spawn_state_(NULL),
-      gc_prologue_callback_(NULL),
-      gc_epilogue_callback_(NULL),
       defer_finalization_count_(0),
       pending_deopts_(new MallocGrowableArray<PendingLazyDeopt>),
       deopt_context_(NULL),
       tag_table_(GrowableObjectArray::null()),
       deoptimized_code_array_(GrowableObjectArray::null()),
       sticky_error_(Error::null()),
       next_(NULL),
       loading_invalidation_gen_(kInvalidGen),
       top_level_parsing_count_(0),
       field_list_mutex_(new Mutex()),
@@ skipping 2018 matching lines @@
 void IsolateSpawnState::DecrementSpawnCount() {
   ASSERT(spawn_count_monitor_ != NULL);
   ASSERT(spawn_count_ != NULL);
   MonitorLocker ml(spawn_count_monitor_);
   ASSERT(*spawn_count_ > 0);
   *spawn_count_ = *spawn_count_ - 1;
   ml.Notify();
 }
 
 }  // namespace dart