Make NavigationOperation a MergeableOperation.

pkg/analysis_server/lib/src/operation/operation_queue.dart

Index: pkg/analysis_server/lib/src/operation/operation_queue.dart
diff --git a/pkg/analysis_server/lib/src/operation/operation_queue.dart b/pkg/analysis_server/lib/src/operation/operation_queue.dart
index ced4a8c9e54f74c59d2c92e382f5b5fc6558d6b0..1687a4bc3209fdde14a7f59a7d712b52d8ac24e4 100644
--- a/pkg/analysis_server/lib/src/operation/operation_queue.dart
+++ b/pkg/analysis_server/lib/src/operation/operation_queue.dart
@@ -39,6 +39,14 @@ class ServerOperationQueue {
   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);
   }
 
@@ -119,7 +127,7 @@ class ServerOperationQueue {
    */
   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;