Use the package_resolver package.

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 'package:boolean_selector/boolean_selector.dart';
 
 import '../../backend/test_platform.dart';
 import '../../frontend/timeout.dart';
 import '../configuration.dart';
 import 'values.dart';
 
 /// The parser used to parse the command-line arguments.
 final ArgParser _parser = (() {
   var parser = new ArgParser(allowTrailingOptions: true);
 
   var allPlatforms = TestPlatform.all.toList();
   if (!Platform.isMacOS) allPlatforms.remove(TestPlatform.safari);
   if (!Platform.isWindows) allPlatforms.remove(TestPlatform.internetExplorer);
 
   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.\n'
@@ skipping 147 matching lines @@
       }
     }
 
     return new Configuration(
         help: _ifParsed('help'),
         version: _ifParsed('version'),
         verboseTrace: _ifParsed('verbose-trace'),
         jsTrace: _ifParsed('js-trace'),
         pauseAfterLoad: _ifParsed('pause-after-load'),
         color: _ifParsed('color'),
-        packageRoot: _ifParsed('package-root'),
         dart2jsPath: _ifParsed('dart2js-path'),
         dart2jsArgs: _ifParsed('dart2js-args') as List<String>,
         precompiledPath: _ifParsed('precompiled'),
         reporter: _ifParsed('reporter'),
         pubServePort: _parseOption('pub-serve', int.parse),
         concurrency: _parseOption('concurrency', int.parse),
         shardIndex: shardIndex,
         totalShards: totalShards,
         timeout: _parseOption('timeout', (value) => new Timeout.parse(value)),
         patterns: patterns,
@@ skipping 28 matching lines @@
   /// information.
   /*=T*/ _wrapFormatException/*<T>*/(String name, /*=T*/ parse()) {
     try {
       return parse();
     } on FormatException catch (error) {
       throw new FormatException('Couldn\'t parse --$name "${_options[name]}": '
           '${error.message}');
     }
   }
 }