Added check that the object being stored as a return value in the NativeArguments array is a Dart i…

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 19928)
+++ runtime/vm/raw_object.h	(working copy)
@@ -341,6 +341,10 @@
     ptr()->tags_ = CreatedFromSnapshotTag::update(true, tags);
   }
 
+  bool IsDartInstance() {
+    return (!IsHeapObject() || (GetClassId() >= kInstanceCid));
+  }
+
   intptr_t Size() const {
     uword tags = ptr()->tags_;
     intptr_t result = SizeTag::decode(tags);