Fixes shutdown crashes.

runtime/vm/service_isolate.cc

Index: runtime/vm/service_isolate.cc
diff --git a/runtime/vm/service_isolate.cc b/runtime/vm/service_isolate.cc
index bd73eb83ddb3e360514f00c6745f101245c35dbf..1957b1b79cfec33fe9c90a8dba1f0a6575ee4de7 100644
--- a/runtime/vm/service_isolate.cc
+++ b/runtime/vm/service_isolate.cc
@@ -301,6 +301,7 @@ class RunServiceTask : public ThreadPool::Task {
     // TODO(johnmccutchan): Support starting up service isolate without embedder
     // provided isolate creation callback.
     if (create_callback == NULL) {
+      ServiceIsolate::SetServiceIsolate(NULL);
       ServiceIsolate::FinishedInitializing();
       return;
     }
@@ -319,6 +320,7 @@ class RunServiceTask : public ThreadPool::Task {
                                                    &error));
     if (isolate == NULL) {
       OS::PrintErr("vm-service: Isolate creation error: %s\n", error);
+      ServiceIsolate::SetServiceIsolate(NULL);
       ServiceIsolate::FinishedInitializing();
       return;
     }