Fix for issue #28287 (Illegal argument in isolate message : (object is a regular Dart Instance))

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index 13757ecbaee8aaf041b4e0b7ba23ad9f7a4d8d12..b073188befe910a935742705964a68ee5d447de9 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -1137,7 +1137,7 @@ void SnapshotReader::ProcessDeferredCanonicalizations() {
       }
       if (newobj.raw() != objref->raw()) {
         ZoneGrowableArray<intptr_t>* patches = backref.patch_records();
-        ASSERT(newobj.IsCanonical());
+        ASSERT(newobj.IsNull() || newobj.IsCanonical());
         ASSERT(patches != NULL);
         // First we replace the back ref table with the canonical object.
         *objref = newobj.raw();