Fix bug in native message writer, the GetMarkedCObjectMark method was shifting teh value incorrectl…

vm/dart_api_message.cc

Index: vm/dart_api_message.cc
===================================================================
--- vm/dart_api_message.cc	(revision 8402)
+++ vm/dart_api_message.cc	(working copy)
@@ -404,7 +404,8 @@
 
 intptr_t ApiMessageWriter::GetMarkedCObjectMark(Dart_CObject* object) {
   ASSERT(IsCObjectMarked(object));
-  intptr_t mark_value = ((object->type & kDartCObjectMarkMask) >> 3);
+  intptr_t mark_value =
+      ((object->type & kDartCObjectMarkMask) >> kDartCObjectTypeBits);
   // An offset was added to object id for making marking object id 0 possible.
   return mark_value - kDartCObjectMarkOffset;
 }