Avoid closing an already closed channel

pkg/analysis_server/lib/src/plugin/plugin_manager.dart

 // Copyright (c) 2017, 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:collection';
 import 'dart:convert';
 import 'dart:io' show Platform, Process, ProcessResult;
 
 import 'package:analysis_server/src/plugin/notification_manager.dart';
@@ skipping 874 matching lines @@
       }
     }
     info.notificationManager
         .handlePluginNotification(info.pluginId, notification);
   }
 
   /**
    * Handle the fact that the plugin has stopped.
    */
   void handleOnDone() {
-    channel.close();
-    channel = null;
+    if (channel != null) {
+      channel.close();
+      channel = null;
+    }
     pluginStoppedCompleter.complete(null);
   }
 
   /**
    * Handle the fact that an unhandled error has occurred in the plugin.
    */
   void handleOnError(List<String> errorPair) {
     // TODO(brianwilkerson) Decide how we want to handle errors.
     info.instrumentationService.logPluginException(
         info.data, errorPair[0], new StackTrace.fromString(errorPair[1]));
@@ skipping 126 matching lines @@
    * The completer that will be used to complete the future when the response is
    * received from the plugin.
    */
   final Completer<Response> completer;
 
   /**
    * Initialize a pending request.
    */
   _PendingRequest(this.method, this.requestTime, this.completer);
 }