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

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index 88f2a1dfe00b1b974e053450d29bb717aaa004ae..b86781b48f6933ebcb69365f36a626f0b10273f8 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -1365,4 +1365,9 @@ class ServerPerformanceStatistics {
    * 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');
 }