Converted Observatory object-common element

runtime/observatory/tests/observatory_ui/mocks/objects/guarded.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 mocks;
+
+class GuardedMock<T> implements M.Guarded {
+  bool get isSentinel => asSentinel != null;
+  bool get isValue => asValue != null;
+  final T asValue;
+  final M.Sentinel asSentinel;
+
+  const GuardedMock.fromValue(this.asValue)
+    : asSentinel = null;
+
+  const GuardedMock.fromSentinel(this.asSentinel)
+    : asValue = null;
+}