Converted Observatory refs elements

runtime/observatory/tests/observatory_ui/mocks/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 mocks;
 
-class SourceLocationMock implements M.SourceLocation {
-  final M.ScriptRef script;
-  final int tokenPos;
-  final int endTokenPos;
+class Sentinel implements M.Sentinel {
+  final M.SentinelKind kind;
+  final String valueAsString;
 
-  const SourceLocationMock({this.script, this.tokenPos, this.endTokenPos});
+  const Sentinel({this.kind: M.SentinelKind.collected,
+                  this.valueAsString: 'sentinel-value'});
 }