Add more user tags to help trackdown analysis server perf bottlenecks.

pkg/analysis_server/lib/src/analysis_server.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 analysis.server;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:math' show max;
 
@@ skipping 1347 matching lines @@
 
   /**
    * The [PerformanceTag] for time spent in server comminication channels.
    */
   static PerformanceTag serverChannel = new PerformanceTag('serverChannel');
 
   /**
    * The [PerformanceTag] for time spent in server request handlers.
    */
   static PerformanceTag serverRequests = new PerformanceTag('serverRequests');
+
+  /**
+   * The [PerformanceTag] for time spent in split store microtasks.
+   */
+  static PerformanceTag splitStore = new PerformanceTag('splitStore');
 }