Add trace_service_pause_events flag and related tracing

runtime/vm/message_handler.cc

 // Copyright (c) 2011, 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/message_handler.h"
 #include "vm/port.h"
 #include "vm/dart.h"
 
 namespace dart {
 
 DECLARE_FLAG(bool, trace_isolates);
-
+DECLARE_FLAG(bool, trace_service_pause_events);
 
 class MessageHandlerTask : public ThreadPool::Task {
  public:
   explicit MessageHandlerTask(MessageHandler* handler)
       : handler_(handler) {
     ASSERT(handler != NULL);
   }
 
   void Run() {
     handler_->TaskCallback();
@@ skipping 201 matching lines @@
     }
 
     // Handle any pending messages for this message handler.
     if (ok) {
       ok = HandleMessages(true, true);
     }
     task_ = NULL;  // No task in queue.
 
     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) {
         OS::Print("[-] Stopping message handler (%s):\n"
                   "\thandler:    %s\n",
                   (ok ? "no live ports" : "error"),
                   name());
         }
         pool_ = NULL;
         run_end_callback = true;
@@ skipping 61 matching lines @@
 
 void MessageHandler::decrement_control_ports() {
   MonitorLocker ml(&monitor_);
 #if defined(DEBUG)
   CheckAccess();
 #endif
   control_ports_--;
 }
 
 }  // namespace dart