Handle vmservice messages while at breakpoint.

runtime/bin/dbg_message.h

Index: runtime/bin/dbg_message.h
===================================================================
--- runtime/bin/dbg_message.h	(revision 31232)
+++ runtime/bin/dbg_message.h	(working copy)
@@ -161,9 +161,17 @@
                   const char* end,
                   int debug_fd);
 
-  // Handle all debug command messages in the queue.
-  void HandleMessages();
+  // Notify an isolate of a pending vmservice message.
+  void Notify();
 
+  // Run a message loop which handles messages as they arrive until
+  // normal program execution resumes.
+  void MessageLoop();
+
+  // Handle any pending debug command messages in the queue.  Return
+  // value indicates whether a resume has been requested.
+  bool HandlePendingMessages();
+
   // Interrupt Isolate.
   void InterruptIsolate();
 
@@ -225,6 +233,9 @@
                                 const char* end,
                                 int debug_fd);
 
+  // Notify an isolate of a pending vmservice message.
+  static void NotifyIsolate(Dart_Isolate isolate);
+
   // Interrupt isolate.
   static bool InterruptIsolate(Dart_IsolateId isolate_id);