add parser parseAsync flag

pkg/analyzer/test/src/task/options_test.dart

 // Copyright (c) 2015, 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 analyzer.test.src.task.options_test;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/source/analysis_options_provider.dart';
 import 'package:analyzer/source/error_processor.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 32 matching lines @@
       optionsProvider.getOptionsFromString(source);
 
   test_configure_bad_options_contents() {
     configureContext('''
 analyzer:
   strong-mode:true # misformatted
 ''');
     expect(analysisOptions.strongMode, false);
   }
 
+  test_configure_enableAsync() {
+    configureContext('''
+analyzer:
+  language:
+''');
+    expect(analysisOptions.enableAsync, true);
+  }
+
+  test_configure_enableAsync_false() {
+    configureContext('''
+analyzer:
+  language:
+    enableAsync: false
+''');
+    expect(analysisOptions.enableAsync, false);
+  }
+
   test_configure_enableGenericMethods() {
     expect(analysisOptions.enableGenericMethods, false);
     configureContext('''
 analyzer:
   language:
     enableGenericMethods: true
 ''');
     expect(analysisOptions.enableGenericMethods, true);
   }
 
@@ skipping 297 matching lines @@
         [AnalysisOptionsWarningCode.UNSUPPORTED_OPTION_WITH_LEGAL_VALUE]);
   }
 
   void validate(String source, List<AnalysisOptionsErrorCode> expected) {
     var options = optionsProvider.getOptionsFromString(source);
     var errors = validator.validate(options);
     expect(errors.map((AnalysisError e) => e.errorCode),
         unorderedEquals(expected));
   }
 }