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

runtime/bin/vmservice/vmservice_io.dart

Index: runtime/bin/vmservice/vmservice_io.dart
diff --git a/runtime/bin/vmservice/vmservice_io.dart b/runtime/bin/vmservice/vmservice_io.dart
index ba8244111ab2a0cc16aeb85d2ef1bd45e9bd19c4..b15aeb673048ab1a3268042193ac2fcf1a4a0e99 100644
--- a/runtime/bin/vmservice/vmservice_io.dart
+++ b/runtime/bin/vmservice/vmservice_io.dart
@@ -39,6 +39,7 @@ _lazyServerBoot() {
 }
 
 Future cleanupCallback() async {
+  shutdownLoaders();
   // Cancel the sigquit subscription.
   if (_signalSubscription != null) {
     await _signalSubscription.cancel();
@@ -88,6 +89,9 @@ _registerSignalHandler() {
 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();
@@ -95,8 +99,6 @@ main() {
     // 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.