Fix ddbg completers

tools/ddbg.dart

 // 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.
 
 // Simple interactive debugger shell that connects to the Dart VM's debugger
 // connection port.
 
 import "dart:convert";
 import "dart:io";
 import "dart:async";
@@ skipping 68 matching lines @@
 
 
 void quitShell() {
   vmSubscription.cancel();
   vmSock.close();
   cmdo.done();
 }
 
 
 Future sendCmd(Map<String, dynamic> cmd) {
-  var completer = new Completer();
+  var completer = new Completer.sync();
   int id = cmd["id"];
   outstandingCommands[id] = completer;
   if (verbose) {
     print("sending: '${JSON.encode(cmd)}'");
   }
   vmSock.write(JSON.encode(cmd));
   return completer.future;
 }
 
 
@@ skipping 737 matching lines @@
               print('Program exited with code $exitCode.');
             }
           });
 
       debuggerMain();
     });
   } else {
     debuggerMain();
   }
 }