add enableAsync to analysis option file flags

pkg/analysis_server/lib/src/domain_analysis.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 domain.analysis;
 
 import 'dart:async';
 import 'dart:core' hide Resource;
 
 import 'package:analysis_server/plugin/analysis/analysis_domain.dart';
@@ skipping 311 matching lines @@
     if (newOptions.generateHints != null) {
       updaters.add((engine.AnalysisOptionsImpl options) {
         options.hint = newOptions.generateHints;
       });
     }
     if (newOptions.generateLints != null) {
       updaters.add((engine.AnalysisOptionsImpl options) {
         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;
       });
     }
     server.updateOptions(updaters);
     return new AnalysisUpdateOptionsResult().toResponse(request.id);
   }
 
   /**
@@ skipping 59 matching lines @@
       context.onResultComputed(descriptor).listen((result) {
         StreamController<engine.ComputedResult> controller =
             controllers[result.descriptor];
         if (controller != null) {
           controller.add(result);
         }
       });
     }
   }
 }