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

runtime/vm/scavenger.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/scavenger.h"
 
 #include <algorithm>
 #include <map>
 #include <utility>
 
@@ skipping 359 matching lines @@
 
 
 Scavenger::~Scavenger() {
   delete to_;
   delete from_;
   delete space_;
 }
 
 
 void Scavenger::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())();
   }
   // Flip the two semi-spaces so that to_ is always the space for allocating
   // objects.
   MemoryRegion* temp = from_;
   from_ = to_;
   to_ = temp;
   top_ = FirstObjectStart();
   resolved_top_ = top_;
   end_ = to_->end();
 }
@@ skipping 15 matching lines @@
     // objects candidates for promotion.
     survivor_end_ = end_;
   }
 
 #if defined(DEBUG)
   VerifyStoreBufferPointerVisitor verify_store_buffer_visitor(isolate, to_);
   heap_->IterateOldPointers(&verify_store_buffer_visitor);
 
   memset(from_->pointer(), 0xf3, from_->size());
 #endif  // defined(DEBUG)
-  if (invoke_api_callbacks) {
-    isolate->gc_epilogue_callbacks().Invoke();
+  if (invoke_api_callbacks && (isolate->gc_epilogue_callback() != NULL)) {
+    (isolate->gc_epilogue_callback())();
   }
 }
 
 
 void Scavenger::IterateStoreBuffers(Isolate* isolate,
                                     ScavengerVisitor* visitor) {
   StoreBuffer* buffer = isolate->store_buffer();
   heap_->RecordData(kStoreBufferEntries, buffer->Count());
 
   // Iterating through the store buffers.
@@ skipping 332 matching lines @@
 
 
 void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(size >= 0);
   external_size_ -= size;
   ASSERT(external_size_ >= 0);
   end_ = Utils::Minimum(to_->end(), end_ + size);
 }
 
 }  // namespace dart