Eliminate busy waiting for debugger connection

runtime/bin/dbg_connection.h

 // Copyright (c) 2012, 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.
 
 #ifndef BIN_DBG_CONNECTION_H_
 #define BIN_DBG_CONNECTION_H_
 
 #include "bin/builtin.h"
 #include "bin/utils.h"
 
@@ skipping 38 matching lines @@
 
   // The socket that is listening for incoming debugger connections.
   // This descriptor is created and closed by a VM thread.
   static int listener_fd_;
 
   // The socket that connects with the debugger client.
   // The descriptor is created by the debugger connection thread and
   // closed by a VM thread.
   static int debugger_fd_;
 
+  // The VM thread waits on this condition variable when it reaches a
+  // breakpoint and the debugger client has not connected yet.
+  static dart::Monitor is_connected_;
+
   static MessageBuffer* msgbuf_;
 
   friend class DebuggerConnectionImpl;
 
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(DebuggerConnectionHandler);
 };
 
 #endif  // BIN_DBG_CONNECTION_H_