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

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
===================================================================
--- runtime/include/dart_api.h	(revision 33620)
+++ runtime/include/dart_api.h	(working copy)
@@ -606,6 +606,25 @@
 DART_EXPORT Dart_Handle Dart_RemoveGcEpilogueCallback(
     Dart_GcEpilogueCallback callback);
 
+/**
+ * Adds garbage collection callbacks (prologue and epilogue).
+ *
+ * \param prologue_callback A function pointer to a prologue callback function.
+ *   A prologue callback function should not be already set when this function
+ *   is called. A NULL value removes the existing prologue callback function
+ *   if any.
+ *
+ * \param epilogue_callback A function pointer to an epilogue callback function.
+ *   An epilogue callback function should not be already set when this function
+ *   is called. A NULL value removes the existing epilogue callback function
+ *   if any.
+ *
+ * \return Success if the callbacks were added.  Otherwise, returns an
+ *   error handle.
+ */
+DART_EXPORT Dart_Handle Dart_SetGcCallbacks(
+    Dart_GcPrologueCallback prologue_callback,
+    Dart_GcEpilogueCallback epilogue_callback);
 
 /*
  * ==========================