Converted Observatory refs elements

runtime/observatory/lib/src/models/objects/sentinel.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+part of models;
+
+enum SentinelKind {
+  /// Indicates that the object referred to has been collected by the GC.
+  collected,
+  /// Indicates that an object id has expired.
+  expired,
+  /// Indicates that a variable or field has not been initialized.
+  notInitialized,
+  /// Indicates that a variable or field is in the process of being initialized.
+  initializing,
+  /// Indicates that a variable has been eliminated by the optimizing compiler.
+  optimizedOut,
+  /// Reserved for future use.
+  free,
+}
+
+abstract class Sentinel {
+  /// What kind of sentinel is this?
+  SentinelKind get kind;
+
+  /// A reasonable string representation of this sentinel.
+  String get valueAsString;
+}