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

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
===================================================================
--- runtime/vm/scavenger.cc	(revision 33596)
+++ runtime/vm/scavenger.cc	(working copy)
@@ -377,8 +377,8 @@
 
 
 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.
@@ -414,8 +414,8 @@
 
   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())();
   }
 }