Tighten the constraint on args.

lib/src/runner/configuration/args.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:io';
 
 import 'package:args/args.dart';
 
 import '../../frontend/timeout.dart';
 import '../../backend/test_platform.dart';
@@ skipping 12 matching lines @@
   parser.addFlag("help", abbr: "h", negatable: false,
       help: "Shows this usage information.");
   parser.addFlag("version", negatable: false,
       help: "Shows the package's version.");
   parser.addOption("package-root", hide: true);
 
   // Note that defaultsTo declarations here are only for documentation purposes.
   // We pass null values rather than defaults to [new Configuration] so that it
   // merges properly with the config file.
 
+  parser.addSeparator("======== Selecting Tests");
   parser.addOption("name",
       abbr: 'n',
       help: 'A substring of the name of the test to run.\n'
           'Regular expression syntax is supported.');
   parser.addOption("plain-name",
       abbr: 'N',
       help: 'A plain-text substring of the name of the test to run.');
   // TODO(nweiz): Support the full platform-selector syntax for choosing which
   // tags to run. In the shorter term, disallow non-"identifier" tags.
   parser.addOption("tags",
       abbr: 't',
       help: 'Run only tests with all of the specified tags.',
       allowMultiple: true);
   parser.addOption("tag", hide: true, allowMultiple: true);
   parser.addOption("exclude-tags",
       abbr: 'x',
       help: "Don't run tests with any of the specified tags.",
       allowMultiple: true);
   parser.addOption("exclude-tag", hide: true, allowMultiple: true);
 
+  parser.addSeparator("======== Running Tests");
   parser.addOption("platform",
       abbr: 'p',
       help: 'The platform(s) on which to run the tests.',
       defaultsTo: 'vm',
       allowed: allPlatforms.map((platform) => platform.identifier).toList(),
       allowMultiple: true);
   parser.addOption("concurrency",
       abbr: 'j',
       help: 'The number of concurrent test suites run.',
       defaultsTo: defaultConcurrency.toString(),
       valueHelp: 'threads');
   parser.addOption("pub-serve",
       help: 'The port of a pub serve instance serving "test/".',
       valueHelp: 'port');
   parser.addOption("timeout",
       help: 'The default test timeout. For example: 15s, 2x, none',
       defaultsTo: '30s');
   parser.addFlag("pause-after-load",
       help: 'Pauses for debugging before any tests execute.\n'
           'Implies --concurrency=1 and --timeout=none.\n'
           'Currently only supported for browser tests.',
       negatable: false);
 
+  parser.addSeparator("======== Output");
   parser.addOption("reporter",
       abbr: 'r',
       help: 'The runner used to print test results.',
       defaultsTo: defaultReporter,
       allowed: allReporters,
       allowedHelp: {
     'compact': 'A single line, updated continuously.',
     'expanded': 'A separate line for each update.',
     'json': 'A machine-readable format (see https://goo.gl/0HRhdZ).'
   });
@@ skipping 79 matching lines @@
   var value = options[name];
   if (value == null) return null;
 
   try {
     return parse(value);
   } on FormatException catch (error) {
     throw new FormatException('Couldn\'t parse --$name "${options[name]}": '
         '${error.message}');
   }
 }