- Make sure to keep the main/control port opened until the

runtime/vm/message_handler.h

Index: runtime/vm/message_handler.h
===================================================================
--- runtime/vm/message_handler.h	(revision 29277)
+++ runtime/vm/message_handler.h	(working copy)
@@ -53,8 +53,15 @@
   // Returns true on success.
   bool HandleOOBMessages();
 
+  // The number of opened control ports is determined whether the isolate has
+  // live ports. An isolate is considered not having any live ports if only
+  // control ports are open.
+  // Usually either 0 or 1.
+  void increment_control_ports();
+  void decrement_control_ports();
+
   // A message handler tracks how many live ports it has.
-  bool HasLivePorts() const { return live_ports_ > 0; }
+  bool HasLivePorts() const { return live_ports_ > control_ports_; }
 
 #if defined(DEBUG)
   // Check that it is safe to access this message handler.
@@ -120,7 +127,8 @@
   Monitor monitor_;  // Protects all fields in MessageHandler.
   MessageQueue* queue_;
   MessageQueue* oob_queue_;
-  intptr_t live_ports_;
+  intptr_t control_ports_;  // The number of open control ports usually 0 or 1.
+  intptr_t live_ports_;  // The number of open ports, including control ports.
   ThreadPool* pool_;
   ThreadPool::Task* task_;
   StartCallback start_callback_;