Remove background finalization.

runtime/observatory/lib/src/service/object.dart

Index: runtime/observatory/lib/src/service/object.dart
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index 8b8c332a220217dfd787468b7655ddf27b362612..3f99b907bb3361b5a81a3f98ef6eca9ce56d3419 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -3139,10 +3139,6 @@ class Thread extends ServiceObject implements M.Thread {
         _kind = M.ThreadKind.markerTask;
         _kindString = 'marker';
         break;
-      case "kFinalizerTask":
-        _kind = M.ThreadKind.finalizerTask;
-        _kindString = 'finalizer';
-        break;
       default:
         assert(false);
     }