add enableAsync to analysis option file flags

pkg/analysis_server/lib/src/domain_analysis.dart

Index: pkg/analysis_server/lib/src/domain_analysis.dart
diff --git a/pkg/analysis_server/lib/src/domain_analysis.dart b/pkg/analysis_server/lib/src/domain_analysis.dart
index 0c8d28bdfee458962cb94deac5d08ff5f5864e38..1ecbbbd720eadd8512f10cf2eaa42b0d9ecb7b72 100644
--- a/pkg/analysis_server/lib/src/domain_analysis.dart
+++ b/pkg/analysis_server/lib/src/domain_analysis.dart
@@ -329,6 +329,11 @@ class AnalysisDomainHandler implements RequestHandler {
         options.lint = newOptions.generateLints;
       });
     }
+    if (newOptions.enableAsync != null) {
+      updaters.add((engine.AnalysisOptionsImpl options) {
+        options.enableAsync = newOptions.enableAsync;
+      });
+    }
     if (newOptions.enableSuperMixins != null) {
       updaters.add((engine.AnalysisOptionsImpl options) {
         options.enableSuperMixins = newOptions.enableSuperMixins;