Added comments explaining why some code is not used / returns empty results as a result of issue #2…

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 8f82cd18dba65007c6bac7bb53831417e786e382..954f24e6a55799ead125c570499d447097bb3dcc 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -3107,6 +3107,8 @@ class Thread extends ServiceObject implements M.Thread {
   String _kindString;
   int get memoryHighWatermark => _memoryHighWatermark;
   int _memoryHighWatermark;
+
+  // TODO(bkonyi): zones will always be empty. See issue #28885.
   List<Zone> get zones => _zones;
   final List<Zone> _zones = new List<Zone>();