Perform override and inheritance checks.

sdk/lib/_internal/lib/isolate_helper.dart

Index: sdk/lib/_internal/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/lib/isolate_helper.dart b/sdk/lib/_internal/lib/isolate_helper.dart
index af5c9d55185f39e4c96c4ea28a46acce19f6f987..39ace99ceb969652287be5bf716c3b87b3250b68 100644
--- a/sdk/lib/_internal/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/lib/isolate_helper.dart
@@ -1043,7 +1043,7 @@ class _MessageTraverserVisitedMap {
 }
 
 /** Abstract visitor for dart objects that can be sent as isolate messages. */
-class _MessageTraverser {
+abstract class _MessageTraverser {
 
   _MessageTraverserVisitedMap _visited;
   _MessageTraverser() : _visited = new _MessageTraverserVisitedMap();
@@ -1167,7 +1167,7 @@ class _Serializer extends _MessageTraverser {
 }
 
 /** Deserializes arrays created with [_Serializer]. */
-class _Deserializer {
+abstract class _Deserializer {
   Map<int, dynamic> _deserialized;
 
   _Deserializer();