call dispose so completion manager can cancel pending operations

pkg/analysis_server/lib/src/services/completion/completion_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.
 
 library services.completion.computer;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
@@ skipping 74 matching lines @@
   /**
    * Compute completion results for the given reqeust and append them to the stream.
    * Clients should not call this method directly as it is automatically called
    * when a client listens to the stream returned by [results].
    * Subclasses should override this method, append at least one result
    * to the [controller], and close the controller stream once complete.
    */
   void computeSuggestions(CompletionRequest request);
 
   /**
+   * Discard any pending operations.
+   * Subclasses may override but should call super.dispose
+   */
+  void dispose() {
+  }
+
+  /**
    * Generate a stream of code completion results.
    */
   Stream<CompletionResult> results(CompletionRequest request) {
     controller = new StreamController<CompletionResult>(onListen: () {
       scheduleMicrotask(() {
         computeSuggestions(request);
       });
     });
     return controller.stream;
   }
@@ skipping 150 matching lines @@
    */
   final String name;
 
   /**
    * The elapse time or `null` if undefined.
    */
   final Duration elapsed;
 
   OperationPerformance(this.name, this.elapsed);
 }