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

runtime/lib/vmservice.cc

Index: runtime/lib/vmservice.cc
diff --git a/runtime/lib/vmservice.cc b/runtime/lib/vmservice.cc
index 502f5107a919cb8be0fb51d2d0c3db2488fef489..9c4fc7fb123a0a732800fb21f7f60f6af3a2793d 100644
--- a/runtime/lib/vmservice.cc
+++ b/runtime/lib/vmservice.cc
@@ -9,6 +9,7 @@
 #include "vm/flags.h"
 #include "vm/growable_array.h"
 #include "vm/message.h"
+#include "vm/message_handler.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/port.h"
@@ -139,6 +140,9 @@ DEFINE_NATIVE_ENTRY(VMService_OnStart, 0) {
 DEFINE_NATIVE_ENTRY(VMService_OnExit, 0) {
   if (FLAG_trace_service) {
     OS::Print("vm-service: processed exit message.\n");
+    MessageHandler* message_handler = isolate->message_handler();
+    OS::Print("vm-service: live ports = %" Pd "\n",
+              message_handler->live_ports());
   }
   return Object::null();
 }