call dispose so completion manager can cancel pending operations

pkg/analysis_server/lib/src/domain_completion.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.
 
 library domain.completion;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
@@ skipping 247 matching lines @@
   }
 
   /**
    * Discard the sourcesChanged subscription if any
    */
   void _discardManager() {
     if (_sourcesChangedSubscription != null) {
       _sourcesChangedSubscription.cancel();
       _sourcesChangedSubscription = null;
     }
-    _manager = null;
+    if (_manager != null) {
+      _manager.dispose();
+      _manager = null;
+    }
   }
 }