gracefully degrade with response to client if fail to access client specified dart sdk

pkg/analysis_server/lib/src/analysis_manager.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:analysis_server/src/channel.dart';
 import 'package:analysis_server/src/domain_server.dart';
@@ skipping 108 matching lines @@
   Future<bool> stop() {
     if (process == null) {
       return channel.close().then((_) => false);
     }
     return channel
         .sendRequest(new Request('0', ServerDomainHandler.SHUTDOWN_METHOD))
         .timeout(new Duration(seconds: 2), onTimeout: () {
           print('Expected shutdown response');
         })
         .then((Response response) {
-          channel.close();
-          return process.exitCode;
+          return channel.close().then((_) => process.exitCode);
         })
         .timeout(new Duration(seconds: 2), onTimeout: () {
           print('Expected server to shutdown');
           process.kill();
         })
         .then((int result) {
           if (result != null && result != 0) {
             exitCode = result;
           }
           return true;
         });
   }
 }