Make NavigationOperation a MergeableOperation.

pkg/analysis_server/lib/src/operation/operation_queue.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.queue;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/operation/operation.dart';
@@ skipping 21 matching lines @@
   }
 
   /**
    * Adds the given operation to this queue. The exact position in the queue
    * depends on the priority of the given operation relative to the priorities
    * of the other operations in the queue.
    */
   void add(ServerOperation operation) {
     int queueIndex = operation.priority.ordinal;
     Queue<ServerOperation> queue = _queues[queueIndex];
+    // try to merge into an existing operation
+    for (ServerOperation existingOperation in queue) {
+      if (existingOperation is MergeableOperation &&
+          existingOperation.merge(operation)) {
+        return;
+      }
+    }
+    // add it
     queue.addLast(operation);
   }
 
   /**
    * Removes all elements in the queue.
    */
   void clear() {
     for (Queue<ServerOperation> queue in _queues) {
       queue.clear();
     }
@@ skipping 60 matching lines @@
       }
     }
     return null;
   }
 
   /**
    * Returns an operation that satisfies the given [test] or `null`.
    */
   ServerOperation takeIf(bool test(ServerOperation operation)) {
     for (Queue<ServerOperation> queue in _queues) {
-      for (var operation in queue) {
+      for (ServerOperation operation in queue) {
         if (test(operation)) {
           queue.remove(operation);
           return operation;
         }
       }
     }
     return null;
   }
 }