Fixes shutdown crashes.

runtime/vm/service_isolate.cc

 // 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.
 
 #include "vm/service_isolate.h"
 
 #include "vm/compiler.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/isolate.h"
@@ skipping 283 matching lines @@
     TimelineDurationScope tds(Timeline::GetVMStream(),
                               "ServiceIsolateStartup");
     char* error = NULL;
     Isolate* isolate = NULL;
 
     Dart_IsolateCreateCallback create_callback =
         ServiceIsolate::create_callback();
     // TODO(johnmccutchan): Support starting up service isolate without embedder
     // provided isolate creation callback.
     if (create_callback == NULL) {
+      ServiceIsolate::SetServiceIsolate(NULL);
       ServiceIsolate::FinishedInitializing();
       return;
     }
 
     Isolate::Flags default_flags;
     Dart_IsolateFlags api_flags;
     default_flags.CopyTo(&api_flags);
 
     isolate =
         reinterpret_cast<Isolate*>(create_callback(ServiceIsolate::kName,
                                                    NULL,
                                                    NULL,
                                                    NULL,
                                                    &api_flags,
                                                    NULL,
                                                    &error));
     if (isolate == NULL) {
       OS::PrintErr("vm-service: Isolate creation error: %s\n", error);
+      ServiceIsolate::SetServiceIsolate(NULL);
       ServiceIsolate::FinishedInitializing();
       return;
     }
 
 
     Thread::ExitIsolate();
 
     ServiceIsolate::ConstructExitMessageAndCache(isolate);
 
     RunMain(isolate);
@@ skipping 160 matching lines @@
   ASSERT(!result.IsNull());
   Dart_Port port = ILLEGAL_PORT;
   if (result.IsReceivePort()) {
     port = ReceivePort::Cast(result).Id();
   }
   ASSERT(port != ILLEGAL_PORT);
   ServiceIsolate::SetServicePort(port);
 }
 
 }  // namespace dart