add enableAsync to analysis option file flags

pkg/analysis_server/test/context_manager_test.dart

Index: pkg/analysis_server/test/context_manager_test.dart
diff --git a/pkg/analysis_server/test/context_manager_test.dart b/pkg/analysis_server/test/context_manager_test.dart
index 082f28df46830835e0ada52be1fee1b72928da6f..f6eff473858c1bfbae4069ddd2b8c4dbca079528 100644
--- a/pkg/analysis_server/test/context_manager_test.dart
+++ b/pkg/analysis_server/test/context_manager_test.dart
@@ -316,6 +316,7 @@ analyzer:
     - 'test/**'
   language:
     enableGenericMethods: true
+    enableAsync: false
   errors:
     unused_local_variable: false
 linter:
@@ -338,6 +339,7 @@ linter:
     // * from `_embedder.yaml`:
     expect(context.analysisOptions.strongMode, isTrue);
     expect(context.analysisOptions.enableSuperMixins, isTrue);
+    expect(context.analysisOptions.enableAsync, isFalse);
     // * from `.analysis_options`:
     expect(context.analysisOptions.enableGenericMethods, isTrue);
     // * verify tests are excluded