call dispose so completion manager can cancel pending operations

pkg/analysis_server/lib/src/services/completion/completion_manager.dart

Index: pkg/analysis_server/lib/src/services/completion/completion_manager.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/completion_manager.dart b/pkg/analysis_server/lib/src/services/completion/completion_manager.dart
index c66da42a8885d3654a83fc6380bcfe79c3786218..a8d16db64cbc37de62bd1182d50e6ff8a9505d78 100644
--- a/pkg/analysis_server/lib/src/services/completion/completion_manager.dart
+++ b/pkg/analysis_server/lib/src/services/completion/completion_manager.dart
@@ -92,6 +92,13 @@ abstract class CompletionManager {
   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) {