Make NavigationOperation a MergeableOperation.

pkg/analysis_server/lib/src/operation/operation.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 operation;
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
+ * [MergeableOperation] can decide whether other operation can be merged into
+ * it, so that it should not be added as a separate operation.
+ */
+abstract class MergeableOperation extends ServerOperation {
+  MergeableOperation(AnalysisContext context) : super(context);
+
+  /**
+   * Attempt to merge the given [other] operation into this one, return `true`
+   * in case of success, so that [other] should not be added as a separate
+   * operation.
+   */
+  bool merge(ServerOperation other);
+}
+
+/**
  * The class [ServerOperation] defines the behavior of objects used to perform
  * operations on a [AnalysisServer].
  */
 abstract class ServerOperation {
   /**
    * The context for this operation.  Operations will be automatically
    * de-queued when their context is destroyed.
    */
   final AnalysisContext context;
 
@@ skipping 51 matching lines @@
  */
 abstract class SourceSensitiveOperation extends ServerOperation {
   SourceSensitiveOperation(AnalysisContext context) : super(context);
 
   /**
    * The given [source] is about to be changed.
    * Check if this [SourceSensitiveOperation] should be discarded.
    */
   bool shouldBeDiscardedOnSourceChange(Source source);
 }