Change output of test.dart --list option, so it can be parsed automatically.

tools/testing/dart/test_options.dart

 // Copyright (c) 2011, 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_options_parser");
 
 #import("dart:io");
 
 List<String> defaultTestSelectors =
     const ['dartc', 'samples', 'standalone', 'corelib', 'co19', 'language',
@@ skipping 78 matching lines @@
               'timeout',
               'Timeout in seconds',
               ['-t', '--timeout'],
               [],
               -1,
               'int'),
           new _TestOptionSpecification(
               'progress',
               'Progress indication mode',
               ['-p', '--progress'],
-              ['compact', 'color', 'line', 'verbose', 'status', 'buildbot'],
+              ['compact', 'color', 'line', 'verbose',
+               'silent', 'status', 'buildbot'],
               'compact'),
           new _TestOptionSpecification(
               'report',
               'Print a summary report of the number of tests, by expectation',
               ['--report'],
               [],
               false,
               'bool'),
           new _TestOptionSpecification(
               'tasks',
@@ skipping 376 matching lines @@
         return option;
       }
     }
     print('Unknown test option $name');
     exit(1);
   }
 
 
   List<_TestOptionSpecification> _options;
 }