Use a tag bit for indicating prologue weak persistent handles, this avoids

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 33182)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -137,11 +137,6 @@
   static FinalizablePersistentHandle* UnwrapAsWeakPersistentHandle(
       Dart_WeakPersistentHandle object);
 
-  // Validates and converts the passed in handle as a prologue weak
-  // persistent handle.
-  static FinalizablePersistentHandle* UnwrapAsPrologueWeakPersistentHandle(
-      Dart_WeakPersistentHandle object);
-
   // Returns an Error handle if isolate is in an inconsistent state.
   // Returns a Success handle when no error condition exists.
   static Dart_Handle CheckIsolateState(Isolate *isolate);