getIsolate rpc now returns collected sentinel for expired isolate.

runtime/observatory/tests/service/get_isolate_rpc_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
@@ skipping 28 matching lines @@
       await vm.invokeRpcNoUpgrade('getIsolate', params);
       expect(false, isTrue, reason:'Unreachable');
     } on ServerRpcException catch(e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
       expect(e.message,
              "getIsolate: invalid 'isolateId' parameter: badid");
     }
     expect(caughtException, isTrue);
   },
+
+  // Plausible isolate id, not found.
+  (VM vm) async {
+    var params = {
+      'isolateId': 'isolates/9999999999',
+    };
+    var result = await vm.invokeRpcNoUpgrade('getIsolate', params);
+    expect(result['type'], equals('Sentinel'));
+    expect(result['kind'], equals('Collected'));
+    expect(result['valueAsString'], equals('<collected>'));
+  },
 ];
 
 main(args) async => runVMTests(args, tests);