Pass --enable-async to the analyzer

tools/testing/dart/test_suite.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 2023 matching lines @@
         args.add("--allow-mock-compilation");
         args.add("--categories=all");
       }
     }
     if ((compiler == "dart2js" || compiler == "dart2dart") &&
         configuration["minified"]) {
       args.add("--minify");
     }
     if (compiler == "dartanalyzer" || compiler == "dart2analyzer") {
       args.add("--show-package-warnings");
+      args.add("--enable-async");
     }
     return args;
   }
 
   static bool isBrowserRuntime(String runtime) {
     const BROWSERS = const [
       'drt',
       'dartium',
       'ie9',
       'ie10',
@@ skipping 233 matching lines @@
  * $pass tests are expected to pass
  * $failOk tests are expected to fail that we won't fix
  * $fail tests are expected to fail that we should fix
  * $crash tests are expected to crash that we should fix
  * $timeout tests are allowed to timeout
  * $compileErrorSkip tests are skipped on browsers due to compile-time error
 """;
     print(report);
   }
 }