Rework standalone to use a synchronous loader that does not invoke Dart code

runtime/bin/vmservice/vmservice_io.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.
 
 library vmservice_io;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
@@ skipping 21 matching lines @@
   if (server != null) {
     return;
   }
   // Lazily create service.
   var service = new VMService();
   // Lazily create server.
   server = new Server(service, _ip, _port);
 }
 
 Future cleanupCallback() async {
+  shutdownLoaders();
   // Cancel the sigquit subscription.
   if (_signalSubscription != null) {
     await _signalSubscription.cancel();
     _signalSubscription = null;
   }
   if (server != null) {
     try {
       await server.cleanup(true);
     } catch (e, st) {
       print("Error in vm-service shutdown: $e\n$st\n");
@@ skipping 29 matching lines @@
   if (_isWindows) {
     // Cannot register for signals on Windows.
     return;
   }
   _signalSubscription = _signalWatch(ProcessSignal.SIGQUIT).listen(_onSignal);
 }
 
 main() {
   // Set embedder hooks.
   VMServiceEmbedderHooks.cleanup = cleanupCallback;
+  // Always instantiate the vmservice object so that the exit message
+  // can be delivered and waiting loaders can be cancelled.
+  var service = new VMService();
   if (_autoStart) {
     _lazyServerBoot();
     server.startup();
     // It's just here to push an event on the event loop so that we invoke the
     // scheduled microtasks.
     Timer.run(() {});
   }
-  // TODO(johnmccutchan): Fixup service isolate shutdown in the general case.
-  // See ServiceIsolate::KillServiceIsolate and ServiceIsolate::Shutdown.
   scriptLoadPort.handler = _processLoadRequest;
   // Register signal handler after a small delay to avoid stalling main
   // isolate startup.
   new Timer(shortDelay, _registerSignalHandler);
   return scriptLoadPort;
 }
 
 _shutdown() native "VMServiceIO_Shutdown";