Add trace_service_pause_events flag and related tracing

runtime/vm/message_handler.cc

Index: runtime/vm/message_handler.cc
diff --git a/runtime/vm/message_handler.cc b/runtime/vm/message_handler.cc
index 32756b8f2489c4ea247ddc29c1ef47daaec5c501..1b841ebfda04788a0b2df750a25aada63a9587ca 100644
--- a/runtime/vm/message_handler.cc
+++ b/runtime/vm/message_handler.cc
@@ -9,7 +9,7 @@
 namespace dart {
 
 DECLARE_FLAG(bool, trace_isolates);
-
+DECLARE_FLAG(bool, trace_service_pause_events);
 
 class MessageHandlerTask : public ThreadPool::Task {
  public:
@@ -231,6 +231,9 @@ void MessageHandler::TaskCallback() {
 
     if (!ok || !HasLivePorts()) {
       if (pause_on_exit()) {
+        if (FLAG_trace_service_pause_events && !paused_on_exit_) {
+          OS::PrintErr("Isolate %s paused before exiting.\n", name());

[keertip, 2014/05/08 17:18:12]

Would it be possible to print out a url too? eg.
http://localhost:54573/#/isolates/796625752.

[Cutch, 2014/05/08 21:54:55]

At startup, yes. At the pause event, it's not possible because we don't have
access to the port or IP address.

The initial printing is done.

+        }
         paused_on_exit_ = true;
       } else {
         if (FLAG_trace_isolates) {