add parser parseAsync flag

pkg/analyzer/lib/src/generated/utilities_general.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 analyzer.src.generated.utilities_general;
 
 import 'dart:collection';
 import 'dart:developer' show UserTag;
 
 /**
+ * Test if the given [value] is `false` or the string "false" (case-insensitive).
+ */
+bool isFalse(Object value) =>
+    value is bool ? !value : toLowerCase(value) == 'false';
+
+/**
  * Test if the given [value] is `true` or the string "true" (case-insensitive).
  */
 bool isTrue(Object value) =>
     value is bool ? value : toLowerCase(value) == 'true';
 
 /**
  * Safely convert this [value] to lower case, returning `null` if [value] is
  * null.
  */
 String toLowerCase(Object value) => value?.toString()?.toLowerCase();
@@ skipping 163 matching lines @@
 
   makeCurrentWhile(f()) {
     PerformanceTag prevTag = makeCurrent();
     try {
       return f();
     } finally {
       prevTag.makeCurrent();
     }
   }
 }