getIsolate rpc now returns collected sentinel for expired isolate.

runtime/vm/service/running_isolates.dart

Index: runtime/vm/service/running_isolates.dart
diff --git a/runtime/vm/service/running_isolates.dart b/runtime/vm/service/running_isolates.dart
index 48ac1f52dda621a6769f1718bc2dca80c4a9a645..b10128c3b1d8791f040bc904fc39412c423d5191 100644
--- a/runtime/vm/service/running_isolates.dart
+++ b/runtime/vm/service/running_isolates.dart
@@ -47,8 +47,14 @@ class RunningIsolates implements MessageRouter {
     }
     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);