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

runtime/vm/gc_marker.cc

 // Copyright (c) 2011, 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/gc_marker.h"
 
 #include <map>
 #include <utility>
 #include <vector>
 
@@ skipping 298 matching lines @@
       handle->UpdateUnreachable(isolate(), is_prologue_weak);
     }
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MarkingWeakVisitor);
 };
 
 
 void GCMarker::Prologue(Isolate* isolate, bool invoke_api_callbacks) {
-  if (invoke_api_callbacks) {
-    isolate->gc_prologue_callbacks().Invoke();
+  if (invoke_api_callbacks && (isolate->gc_prologue_callback() != NULL)) {
+    (isolate->gc_prologue_callback())();
   }
   // The store buffers will be rebuilt as part of marking, reset them now.
   isolate->store_buffer()->Reset();
 }
 
 
 void GCMarker::Epilogue(Isolate* isolate, bool invoke_api_callbacks) {
-  if (invoke_api_callbacks) {
-    isolate->gc_epilogue_callbacks().Invoke();
+  if (invoke_api_callbacks && (isolate->gc_epilogue_callback() != NULL)) {
+    (isolate->gc_epilogue_callback())();
   }
 }
 
 
 void GCMarker::IterateRoots(Isolate* isolate,
                             ObjectPointerVisitor* visitor,
                             bool visit_prologue_weak_persistent_handles) {
   isolate->VisitObjectPointers(visitor,
                                visit_prologue_weak_persistent_handles,
                                StackFrameIterator::kDontValidateFrames);
@@ skipping 157 matching lines @@
   MarkingWeakVisitor mark_weak;
   IterateWeakRoots(isolate, &mark_weak, invoke_api_callbacks);
   mark.Finalize();
   ProcessWeakTables(page_space);
   ProcessObjectIdTable(isolate);
 
   Epilogue(isolate, invoke_api_callbacks);
 }
 
 }  // namespace dart