add enableAsync to analysis option file flags

pkg/analysis_server/test/context_manager_test.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 test.context.directory.manager;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/context_manager.dart';
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 298 matching lines @@
 
     // Setup .analysis_options
     newFile(
         [projPath, AnalysisEngine.ANALYSIS_OPTIONS_FILE],
         r'''
 analyzer:
   exclude:
     - 'test/**'
   language:
     enableGenericMethods: true
+    enableAsync: false
   errors:
     unused_local_variable: false
 linter:
   rules:
     - camel_case_types
 ''');
 
     // Setup context.
     manager.setRoots(<String>[projPath], <String>[], <String, String>{});
     await pumpEventQueue();
 
     // Confirm that one context was created.
     var contexts =
         manager.contextsInAnalysisRoot(resourceProvider.newFolder(projPath));
     expect(contexts, isNotNull);
     expect(contexts, hasLength(1));
     var context = contexts[0];
 
     // Verify options.
     // * 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
     expect(callbacks.currentContextFilePaths[projPath].keys,
         ['/my/proj/sdk_ext/entry.dart']);
 
     // Verify filter setup.
     expect(errorProcessors, hasLength(2));
 
     // * (embedder.)
@@ skipping 2065 matching lines @@
 class TestUriResolver extends UriResolver {
   Map<Uri, Source> uriMap;
 
   TestUriResolver(this.uriMap);
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     return uriMap[uri];
   }
 }