call dispose so completion manager can cancel pending operations

pkg/analysis_server/lib/src/domain_completion.dart

Index: pkg/analysis_server/lib/src/domain_completion.dart
diff --git a/pkg/analysis_server/lib/src/domain_completion.dart b/pkg/analysis_server/lib/src/domain_completion.dart
index c6accf82fd6fb8844f1c309829babc000b0dd162..06c518864c36fff5b531cbcf05cb669c6130bfcb 100644
--- a/pkg/analysis_server/lib/src/domain_completion.dart
+++ b/pkg/analysis_server/lib/src/domain_completion.dart
@@ -265,6 +265,9 @@ class CompletionDomainHandler implements RequestHandler {
       _sourcesChangedSubscription.cancel();
       _sourcesChangedSubscription = null;
     }
-    _manager = null;
+    if (_manager != null) {
+      _manager.dispose();
+      _manager = null;
+    }
   }
 }