Don't quit debugger when the program terminates, only after `quit`.

tests/debugger/async_test.dart

 // Copyright (c) 2015, the Dartino 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.md file.
 
 // Test that we get a meaningful stack trace when called from the
 // event loop.
 
-// DartinoDebuggerCommands=b breakHere,r,bt,t internal,bt,c
+// DartinoDebuggerCommands=b breakHere,r,bt,t internal,bt,c,q
 
 import 'dart:async';
 
 void breakHere() { }
 
 main() {
   new Future.delayed(new Duration(milliseconds: 1), breakHere);
 }