getIsolate rpc now returns collected sentinel for expired isolate.

runtime/vm/service/running_isolates.dart

 // Copyright (c) 2013, 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 vmservice;
 
 class RunningIsolates implements MessageRouter {
   final Map<int, RunningIsolate> isolates = new Map<int, RunningIsolate>();
   int _rootPortId;
 
@@ skipping 29 matching lines @@
       try {
         isolateId = int.parse(isolateParam);
       } catch (e) {
         message.setErrorResponse(
             kInvalidParams, "invalid 'isolateId' parameter: $isolateParam");
         return message.response;
       }
     }
     var isolate = isolates[isolateId];
     if (isolate == null) {
-      message.setErrorResponse(
-          kInvalidParams, "invalid 'isolateId' parameter: $isolateParam");
+      // There is some chance that this isolate may have lived before,
+      // so return a sentinel rather than an error.
+      var result = {
+        'type' : 'Sentinel',
+        'kind' : 'Collected',
+        'valueAsString' : '<collected>',
+      };
+      message.setResponse(encodeResult(message, result));
       return message.response;
     }
     return isolate.route(message);
   }
 }